THE  UNIVERSITY 

OF  ILLINOIS 

LIBRARY 


—  0-1096 


FIELD  MUSEUM  OF  NATURAL  HISTORY 

PUBLICATION  224 
ZOOLOGICAL  SERIES  VOL.  XIV,  No.  3 


THE  BRAINS  OF  THE  SOUTH  AMERICAN 

MARSUPIALS  CAENOLESTES 

AND  OROLESTES 


BY 


JEANNETTE  BROWN  OBENCHAIX 

Hull  Laboratory  of  Anatomy,  University  of  Chicago 


WILFRED  H.  Oscoon 
Curator,  Department  of  Zoology 


CHICAGO,  U.  S.  A 

January  26,  1925 

TBt  U8HAHT  Of  THf 

APR  9     1925 
UNIVERSITY  «r  ILUNM8 


LiKKAKT  !J'   '! 
OCT  2  8  1924 

UNIVERSITY  OF  ILLINOIS 


FIELD  MUSEUM  OF  NATURAL  HISTORY 

PUBLICATION  224 
ZOOLOGICAL  SERIES  VOL.  XIV,  No.  3 


THE  BRAINS  OF  THE  SOUTH  AMERICAN 

MARSUPIALS  CAENOLESTES 

AND  OROLESTES 


BY 


JEANNETTE  BROWN  OBENCHAIN 
Hull  Laboratory  of  Anatomy,  University  of  Chicago 


WILFRED  H.  OSGOOD 
Curator,  Department  of  Zoology 


CHICAGO,  U.  S.  A. 

January  26,  1925 

THE  UflfiAKT  OF  TBf 

APR  9     1925 
UNIVERSITY  «F  ILLINOIS 


THE  BRAINS  OF  THE  SOUTH  AMERICAN 

MARSUPIALS  CAENOLESTES 

AND  OROLESTES 


BY    JEANNETTE    BROWN    OBENCHAIN 


CONTENTS 

PAGE 

Introduction 175 

External  anatomy  of  the  brains  of  Caenolesies  and  Orolestes 178 

Internal  anatomy  of  the  cerebral  hemisphere  of  Caenolestes 188 

Primary  olfactory   area 189 

Secondary  olfactory   areas 189 

Anterior  olfactory  nucleus 191 

Tuberculum  olfactorium 195 

Septal  region 199 

Pyriform  lobe 200 

Hippocampal    formation 206 

Corpus  striatum 220 

Neopallium 221 

General  considerations 222 

A  Summary 225 

^  Acknowledgments        227 

Bibliography 228 

Abbreviations 231 

^ 

INTRODUCTION 

Caenolestes  and  Orolestes  are  tiny  shrewlike  marsupials  five  inches 

*'m  length  of  head  and  body  exclusive  of  the  slender  tail.     They  are 

Natives  of  high  Andean  forests  from  Venezuela  southward  into  Peru, 

.^and  although  discovered  in   1863,  have  not  been  known  to  science, 

CA  except  by  imperfect  material,  until  recent  years.    Their  peculiar  denti- 

tion  has  made  their  assignment  to  one  or  the  other  of  the  marsupial 

suborders  (Diprotodontia  and  Polyprotodontia)  a  matter  of  much  un- 

^  certainty.    This  question  remains  even  yet,  perhaps,  in  suspense,  since 

1*  the  one  brain  character  of  evidential  value  seems  to  reverse  the  latest 

^conclusion,   based   on   careful    sifting   of    all   non-nervous   characters 

^  (Osgood,  1921;  Obenchain,  1923). 

175 


176  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

Aside  from  its  bearing  upon  classification,  the  study  of  the  brains 
of  these  species  is  warranted  by  many  other  considerations.  These 
small  marsupials,  surviving  members  of  an  ancient  American  group, 
are  characterized  by  the  retention  of  many  primitive  features,  by  the 
absence  of  any  great  degree  of  specialization,  by  few  non-marsupial 
characters,  by  numerous  resemblances  to  the  modern  peramelids  (Aus- 
tralian polyprotodonts),  and  by  lack  of  affinity  to  the  other  living 
American  marsupials,  the  polyprotodont  Didelphiidae  (Osgood,  1921, 
pp.  150-151). 

Material. — The  material  which  forms  the  basis  of  the  present  study 
consists  of  the  brains  of  three  female  specimens,  one  of  Caenolestes 
obscurus  and  two  of  Orolestes  inca.  The  first  brain  came  from  an 
adult  female  of  Caenolestes  collected  in  1911  in  the  Colombia- Venezuela 
boundary  region  by  Dr.  Wilfred  H.  Osgood  of  the  Field  Museum  of 
Natural  History;  it  was  examined  by  Dr.  C.  Judson  Herrick,  who 
described  and  figured  its  external  surface  in  a  short  paper  (1921,  antea, 
pp.  157-162,  3  figures)  appended  to  Dr.  Osgood's  long  monograph 
on  the  anatomy  and  zoological  position  of  Caenolestes.  Dr.  Herrick 
later  offered  me  the  opportunity  of  studying  this  brain,  if  I  could  con- 
vert it  into  serial  sections.  The  other  two  brains,  from  adult  females 
of  Orolestes  inca,  were  collected  by  Mr.  E.  Heller  of  the  Yale 
National  Geographic  Society  Peruvian  Expedition  of  1914-15,  and 
were  loaned  by  the  U.  S.  National  Museum  to  the  Field  Museum  to 
be  transferred  to  me  for  the  purpose  of  this  study.  The  first  specimen 
alone  (Caenolestes)  has  been  sectioned  and  stained.  The  description 
of  the  internal  anatomy  is  therefore  based  entirely  on  this  one  trans- 
verse series,  which,  although  somewhat  imperfect  because  of  incom- 
plete fixation  within  the  unopened  skull,  has  stained  with  unexpected 
brilliancy  by  the  iron-haematoxylin  method.  This  series,  owing  to 
the  fact  that  both  cells  and  fibers  have  been  stained,  serves  remarkably 
well,  all  things  considered,  for  the  study  here  undertaken.  Since  the 
brain  was  sectioned  whole,  its  medial  surface  is  known  only  by  means 
of  wax  and  linear  reconstructions  made  from  the  sections.  These  are 
believed  to  be  reasonably  trustworthy,  but  since  they  are  made  from 
an  imperfect  series,  additional  corroboration  was  greatly  to  be  desired. 
Therefore  the  two  brains  of  Orolestes  were  especially  welcome.  I 
take  this  opportunity  to  thank  Mr.  W.  J.  Owen  of  the  Anatomical 
Institute  of  Melbourne,  Australia,  for  his  kind  assistance  in  the  at- 
tempt to  remove  them  as  nearly  intact  as  possible  from  the  skulls.  The 
smaller  of  the  two  brains  (No.  194948)  was  divided  by  a  sagittal  cut, 
the  right  hemisphere  was  detached  from  the  brain  stem,  and  drawings 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      177 

of  the  medial  surface  of  both  the  whole  brain  and  the  detached  hemi- 
sphere were  made  by  Mr.  Kenji  Toda,  of  the  University  of  Chicago, 
to  whom  I  am  indebted  for  the  five  beautiful  and  accurate  air  brush 
pictures  of  the  brain  of  Orolestes  (Figs.  1-5).  The  larger  brain  (No. 
194921)  was  used  for  the  external  views,  with  correction  and  supple- 
ment from  the  other  where  necessary,  as  in  the  case  of  the  parafloccular 
lobes,  which  the  larger  brain  had  lost.  The  business  of  clearing  these 
small  and  fragile  brains  from  membranes  and  coagulated  cerebro-spinal 
fluid  was  both  tedious  and  difficult,  but  fairly  successful  in  the  end, 
and  the  figures  are  offered  as  faithful  representations  of  the  actual 
specimens. 

Three  air  brush  drawings  of  the  brain  of  Caenolestes  by  Mr.  A.  B. 
Streedain,  formerly  of  the  Department  of  Anatomy,  also  may  be 
consulted  (Herrick,  antea,  pis.  XXI-XXII).  This  brain,  owing  to 
the  difficulty  of  removing  membranes  and  debris,  was  in  somewhat 
less  favorable  condition  for  drawing  than  were  the  other  two.  This 
applies  particularly  to  the  ventral  surface,  which  was  further  marred 
by  a  considerable  hole  due  to  faulty  fixation. 

All  three  specimens  were  originally  preserved  whole,  with  only  a 
ventral  longitudinal  opening  of  the  body,  in  10%  formalin,  and  sub- 
sequently transferred  to  alcohol.  In  the  case  of  Caenolestes,  the  brain, 
when  freed  from  the  skull,  ballooned  out  with  a  glistening  white  sur- 
face, but  later  shrunk  and  darkened.  The  other  two  brains  (Oroles- 
tes) were  quite  rigid  when  uncovered,  and  presented  a  dull,  light  brown 
surface.  All  three  completely  filled  the  skull.  It  is  hoped  that  the 
two  brains  of  Orolestes  may  be  successfully  converted  into  sagittal  and 
horizontal  series. 

Method. — The  brain  of  Caenolestes,  infiltrated  in  42  degres  paraf- 
fine  and  blocked  in  56  degree  paraffine  (to  minimize  brittleness),  was 
cut  10  micra  anteriorly  and  15  micra  farther  back,  and  made  about 
1 200  sections.  About  850  sections  were  drawn  at  a  magnification  of 
25  diameters  with  the  aid  of  the  Edinger  projection  apparatus.  These 
have  proved  a  very  useful  close  series  for  annotation.  A  wax  model 
made  from  these  drawings,  while  somewhat  disappointing,  especially 
in  the.  midregion,  owing  to  the  emptiness  of  the  greatly  expanded  ven- 
tricles (the  fragility  of  the  specimen  precluded  aspiration)  and  conse- 
quent wrinkling  and  uneven  spreading  of  the  thinner  portions  of  the 
walls,  has  been  an  invaluable  aid  to  study.  Linear  reconstructions 
drawn  to  scale  have  also  proved  indispensable.  An  additional  series 
of  31  Edinger  drawings  with  a  magnification  85  was  made  from 
the  precommissural  hippocampal  region. 


178  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

EXTERNAL  ANATOMY 

Dr.  Herrick,  in  the  first  account  ever  given  of  the  brain  of  Caeno- 
lestes  (or  of  any  caenolestid  brain),  emphasized  its  extreme  simplicity, 
the  enormous  development  of  its  rhinencephalon,  the  smoothness  and 
limited  extent  of  its  neopallium,  and  called  attention  to  the  fact  that 
it  most  closely  resembles  the  brains  of  the  lowly  Australian  polyproto- 
donts,  Notoryctes  and  Perameles.  To  this  it  will  be  necessary  to  add 
the  description  of  the  median  section  of  the  whole  brain  and  median 
surface  of  the  hemisphere,  as  well  as  a  discussion  of  the  other  sur- 
faces in  the  light  of  knowledge  of  the  internal  structure  of  Caenolestes, 
with  reference  to  changes  of  nomenclature,  or  in  comparison  with  the 
brain  of  Orolestes.  This  will  of  course  involve  some  unavoidable  repe- 
tition. 

The  two  types,  Caenolestes  and  Orolestes,  exhibit  only  minor  dif- 
ferences in  external  anatomy.  Although  all  three  brains  are  supposed 
to  have  come  from  adult  female  specimens,  they  differ  somewhat  in  size 
and  proportions,  as  may  be  observed  in  the  table  of  measurements. 
Caenolestes  is  the  smallest  of  all,  but  it  has  the  longest  hemisphere. 
Its  cerebellum  is  smaller  and  less  plump  than  the  other  two,  and  ex- 
hibits an  extra  furrow  and  convolution — not  a  matter  of  great  impor- 
tance, as  will  be  seen  below  under  the  description  of  the  cerebellum. 
It  is,  however,  to  be  remembered  that  the  three  brains  represent  two 
genera  of  caenolestids. 

The  olfactory  bulbs  are  truly  enormous,  almost  half  the  total  length 
of  the  hemisphere  at  the  medio-ventral  angle.  They  are  of  the  "sessile" 
type,  with  no  visible  olfactory  peduncle  in  the  intact  brain.  Following 
Livini  (1908),  we  have  called  the  deep  constricting  sulcus  which  marks 
the  posterior  border  of  the  olfactory  formation,  the  fissura  circularis 
(fs.circ.,  Figs,  6,  12,  15,  25-28;  unlabelled,  Fig.  1-5).  The  circu- 
lar fissure  is  a  compound  structure  whose  component  parts  are  homol- 
ogous to  portions  of  fissures  described  in  the  brains  of  other  animals. 
Medially  it  represents  the  anterior  portion  of  the  fissura  prima  of  His, 
above  the  antero-medial  tip  of  the  rhinal  arcuate  fissure  (fs.rh.acr., 
Figs.  12,  28)  ;  dorsally,  after  meeting  the  medial  prolongation  of  the 
rhinal  fissure  (fs.rh.a.,  Figs.  15,  25)  at  its  medial  end,  it  diverges  from 
the  latter  to  leave  the  interval  for  an  exposed  triangle  of  dorsal  olfac- 
tory peduncle,  and  then  meets  the  rhinal  fissure  again  as  the  latter 
turns  caudad  on  the  lateral  stirface  of  the  brain  (Fig.  14)  ;  meets  also 
the  anterior  end  of  the  endorhinal  fissure  (fs.erh.,  Fig.  13)  just  below, 
and  then  drops  downward  across  the  wide  lateral  olfactory  tract  (tr. 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      179 

ol,L,  Fig.  13)  overlying  the  pyriform  lobe,  to  continue  as  a  deep  con- 
striction across  the  base  of  the  brain  between  the  olfactory  bulb  (b.ol. 
and  the  olfactory  tubercle  (t.ol.,  Fig.  15),  finally  joining  the  anterior 
end  of  the  medial  portion  of  the  rhinal  arcuate  fissure  (Fig.  15),  and 
thus  completely  defining  the  caudal  border  of  the  olfactory  formation. 
Dorsally  the  olfactory  bulb  is  overhung  by  the  projecting  frontal  pole 
of  the  hemisphere,  which  conceals  the  exposed  olfactory  peduncle  from 
view.  The  rest  of  the  olfactory  peduncle  (see  anterior  olfactory 
nucleus,  page  191)  is  thrust  deep  into  the  heart  of  the  olfactory  bulb, 
as  recognized  long  ago  in  this  type  of  brain  by  Elliott  Smith  and  others. 
This  condition  is  partly  the  result  of  the  great  caudal  extension  of 
the  olfactory  bulb  itself,  partly  one  of  the  numerous  expressions  of 
the  very  considerable  fore-and-aft  "telescoping"  characteristic  of  this 
brain,  which  puts  it  in  rather  marked  contrast  to  more  elongated  brains 
like  that  of  the  Virginia  opossum  (Didelphis  virginiana),  for  example. 
Didelphis  marsupialis,  however,  resembles  the  caenolestid  brain  in  this 
respect  (Beccari,  1910,  Fig.  18). 

The  great  hemispherical  mass  of  the  tuberculum  olfactorium  (t.ol.} 
is  also  delimited  by  an  encircling  sulcus,  the  fissura  rlrinalis  arcuata 
(fs.rh.arc.,  an  adaptation  of  one  of  Retzius'  names;  see  Herrick, 
19243).  Medially  the  fissure  is  homologous  to  the  caudal  portion  of  the 
fissura  prima  of  His.  Its  posterior  more  transverse  portion  has  been 
called  the  fissura  diagonalis  by  Beccari  (1910),  but  his  median  continu- 
ation of  it  upward  in  the  septum  is  not  followed  here  (see  tuberculum 
olfactorium,  page  196).  Laterally  the  rhinal  arcuate  fissure  is  usually 
taken  to  be  homologous  to  the  endorhinal  fissure,  but  this  is  not  strictly 
accurate,  since  the  endorhinal  fissure  lies  wholly  within  the  pyriform 
lobe  and  therefore  above  the  tuberculum,  as  has  been  noted  in  other 
forms  by  Smith,  Johnson  and  others.  The  endorhinal  fissure 
really  marks  the  dorsal  border  of  the  massive  part  of  the  lateral 
olfactory  tract,  which  is  very  wide  rostrally,  but  rapidly  diminishes 
caudally  through  the  loss  of  fibers  to  the  pyriform  lobe  and  tuberculum 
olfactorium,  so  that  in  its  caudo-ventral  course  it  gradually  approaches 
the  rhinal  arcuate  fissure  and  finally  meets  it  near  the  posterior  limit 
of  the  tuberculum  (Figs.  13,  15,  33-34).  The  greatly  reduced  mas- 
sive part  of  the  lateral  olfactory  tract  lies  at  that  level  within  the 
endorhinal  fissure,  where  the  latter  fuses  with  the  rhinal  arcuate  fissure 
and  so  comes  to  an  end.  In  its  caudo-ventral  course  the  lateral  ol- 
factory tract  leaves  a  thin  film  of  fibers  (tr.ol.l.d.,  Figs.  25-33)  cover- 
ing the  pyriform  cortex  almost  to  the  height  of  the  rhinal  fissure  and 
far  toward  the  caudal  pole  of  the  pyriform  lobe. 


180  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

Another  important  fissure  visible  on  the  lateral  surface  of  the  hemi- 
sphere of  these  brains  is  the  rhinal  fissure  (fs.rh.,  to  be  seen  in  the 
majority  of  the  figures),  which  separates  the  dorsal  or  neopallial  con- 
vexity from  the  latero-ventral  pyriform  lobe.  It  traces  an  almost 
exactly  horizontal  course  backward  over  the  lateral  surface  of  the  brain 
at  the  very  high  level  characteristic  of  the  lowly  marsupials  Notoryctes 
(where  it  is  internal  only)  and  Perameles  and  the  insectivore  Erina- 
ceus,  forms  whose  neopallial  development  is  the  least  extensive  among 
mammals.  Anteriorly  the  rhinal  fissure  is  a  continuation  of  the  deep 
groove  between  the  projecting  frontal  pole  of  the  neopallium  and  the 
exposed  dorsal  part  of  the  olfactory  peduncle  (fissura  rhinalis  medialis 
of  some  writers,  but  here  called  the  fissura  rhinalis  anterior,  fs.rh.a., 
Figs.  12-15),  which  bends  sharply  on  to  the  lateral  surface  of  the 
hemisphere  just  above  the  rostral  end  of  the  endorhinal  fissure  and 
runs  horizontally  backward  at  this  high  level  to  round  the  caudal  pole 
of  the  hemisphere,  where  it  appears  upon  the  median  surface  as  the 
median  rhinal  fissure  running  obscurely  forward  into  the  subicular 
border  of  the  hippocampus  (fs.rh.m.,  Figs.  6,  12,  42-43;  rh.,  Fig.  5, 
where  it  probably  coincides  only  caudally  with  the  diagonal  groove 
shown — there  has  been  no  control  with  sections  in  Orolestes  so  far). 
Even  where  imperfectly  developed  externally  the  position  of  the  fis- 
sura rhinalis  is  unmistakably  discernible  throughout,  and  internally 
it  is  in  Caenolestes  always  clearly  defined. 

The  dorsal  convexity  or  neopallium  contains  only  one  other  ex- 
ternally visible  fissure.  In  Caenolestes,  "about  one-fifth  of  the  dis- 
tance from  the  frontal  to  the  posterior  pole  of  the  hemisphere  there 
is  a  distinct,  though  shallow,  transverse  sulcus  which  probably  repre- 
sents the  sulcus  orbitalis  of  Elliot  Smith's  descriptions  (Herrick, 
1921,  antea,  p.  158).  Ventrally  the  orbital  fissure  is  "obscurely  con- 
fluent" with  the  rhinal  fissure.  In  the  smaller  of  the  two  brains  of 
Orolestes  this  fissure  seems  to  be  absent  as  an  external  groove,  but 
its  position  is  apparently  marked  by  blood  vessels.  In  the  reconstruc- 
tions of  Caenolestes  also  it  does  not  show  up,  and  a  cursory  examina- 
tion of  the  sections  has  failed  to  reveal  it.  A  more  careful  study 
might  disclose  its  position. 

The  pyriform  lobe  comprises  the  larger  part  of  the  lateral  and  ven- 
tral aspects  of  the  hemisphere.  Its  dorsal  boundary  is  the  rhinal  fis- 
sure, which  is  at  the  same  time  the  latero-ventral  boundary  of  the  neo- 
pallium. Antero-laterally  the  pyriform  lobe  contains  the  very  sharp 
endorhinal  fissure  (as  explained  above,  page  179)  and  below  this  fis- 
stire  its  antero-lateral  boundary  is  the  lateral  portion  of  the  rhinal 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      181 

arcuate  fissure  (sharply  incised  in  Orolestes  but  not  depressed  at  all 
in  Caenolestes) ,  a  fissure  marking  the  ventral  border  of  the  massive 
portion  of  the  lateral  olfactory  tract,  as  the  boundary  of  the  tuber- 
culum,  since  the  latter  appears  just  ventral  to  this  line. 

The  greatly  widened  caudal  portion  of  the  pyriform  lobe  is  divided 
into  two  distinct  parts  by  a  very  important  fissure  so  faintly  indicated 
externally  that  its  existence  was  unsuspected  before  the  brain  was 
sectioned.  This  is  the  fissura  amygdaloidea  (1913,  1923)  of  Johnston 
(fs.amg.,  Figs  2,  3,  6,  12,  13,  15,  36-43).  In  the  sections  it  branches 
quite  clearly  from  the  rhinal  arcuate  fissure  a  short  distance  in  front 
of  the  caudal  pole  of  the  tuberculum  (between  Figs.  35  and  36),  and 
very  soon  disappears  as  a  recognizable  fissure.  Internally  it  is  quite 
distinct  throughout  its  course,  marking  the  quasi -horizontal  line  of 
junction  between  the  sharply  defined  ventral  edge  of  the  pyriform  cor- 
tex and  the  lateral  border  of  the  amygdaloid  complex  which  occupies 
practically  the  entire  base  of  the  hemisphere  behind  the  tuberculum. 
The  rather  pronounced  sculpturing  of  this  region  is  due  to  the  state  of 
development  of  the  different  amygdaloid  nuclei,  which  have  been  so 
thoroughly  worked  out  in  the  Virginia  opossum  by  Johnston  (1923). 
These  will  be  more  fully  described  further  on,  in  connection  with  the 
internal  anatomy  of  this  brain  (page  201). 

In  Caenolestes  a  very  salient  caudo-ventro-lateral  prominence  cor- 
responds to  the  "natiform  eminence"  of  Elliot  Smith  (i895b)  in 
Notoryctes  (em.nat.,  Figs.  37-41 ;  unlabelled,  Fig.  2,  pi.  XXI,  antea, 
Herrick;  cf.  Orolestes,  figure  3  here,  which  shows  it  much  less  dis- 
tinctly). It  is  due  both  to  an  actual  thickening  of  the  lateral  wall  of 
the  hemisphere  in  this  region  and,  in  greater  degree  perhaps,  to  the 
lateral  cupping  of  the  hemisphere  to  accommodate  the  wide  midbrain. 
Elliot  Smith  thinks  that  the  temporal  bending  of  the  hemisphere  has 
been  a  main  factor  in  the  formation  of  this  eminence.  This  is  much 
more  strongly  suggested  in  Notoryctes  than  in  Caenolestes  (see  page 

213)- 

The  median  section  of  the  brain  of  Caenolestes  is  known  only  in 
wax  and  linear  reconstructions,  but  the  smaller  of  the  two  brains  of 
Orolestes  was  divided  sagittally,  and  figures  4  and  5  show  respectively 
the  left  median  section  of  the  whole  brain  and  the  median  surface  of 
the  right  hemisphere.  Figure  6  is  a  linear  reconstruction  of  the- left 
hemisphere  (reversed)  of  Caenolestes. 

The  third  ventricle  and  external  medial  surface  of  the  brain  of 
Orolestes  were  so  filled  or  coated  with  a  hard  coagulum  of  the  same 
color  as  the  tissue  and  so  tightly  adherent  to  it  that  it  was  not  found 


iS>2  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

possible  to  remove  it  without  some  laceration  of  the  tissue.  Thus  the 
edges  of  the  massa  intermedia  (vn.i.,  Fig.  4)  were  somewhat  frayed, 
the  aqueduct  was  probably  widened,  and  the  epiphyseal  attachments 
and  choroid  roof  of  the  third  ventricle  were  torn.  But  these  defects, 
regrettable  as  they  are,  do  not  greatly  impair  the  value  of  the  section 
for  present  purposes.  At  least  the  sacrifice  of  the  other  brain  with 
the  prospect  of  no  better  success  did  not  in  the  circumstances  seem 
to  be  justified. 

The  large  anterior  or  ventral  commissure  of  Orolestes  (y.,  Figs. 
4  and  5)  is  slightly  oval  and  considerably  darker  than  the  rest  of 
the  surface,  standing  out  sharply.  That  of  Caenolestes  (v.,  Fig.  6)  is 
similar  in  size,  shape  and  position.  The  dorsal  commissure  (rf.)  of 
Orolestes  appears  as  a  moderately  darker  and  distinctly  bilaminar 
mass  occupying  the  dorsal  and  posterior  borders  of  this  portion  of 
the  lamina  terminalis  (lamina  supraneuroporica  of  Johnston),  en- 
closing a  small  whitish  triangular  space  which  gives  the  impression  of 
being  without  transverse  fibers.  The  dorsal  commissure  of  Orolestes 
(counting  it  only  as  the  darkened  bilaminar  mass)  thus  exhibits  mac- 
roscopically  the  typical  marsupial  form,  while  that  of  Caenolestes 
(d.,  Fig.  6)  as  reconstructed  from  sections,  shows  only  the  merest 
hint  of  bilaminarity  in  the  slightly  reentering  anterior  angle  and  the 
dorso-caudal  prolongation  ("splenium"),  suggesting  an  intermediate 
type  between  the  solid  rounded  type  of  the  monotremes  and  the  bilam- 
inar type  of  the  marsupials.  The  reconstruction  of  Caenolestes  was 
made  from  the  exact  midline  of  the  sections,  the  outline  enclosing  only 
the  area  through  which  commiss'ural  fibers  were  coursing.  These  fibers 
are  much  less  dense  in  the  ventral  than  in  the  dorsal  region  of  the 
commissure,  owing  to  the  intermingling  of  many  cells  (bed  or  nucleus 
of  the  dorsal  commissure)  with  the  fibers  in  its  ventral  region. 

In  Orolestes  the  third  ventricle  (Fig.  4),  as  seen  from  the  median 
surface,  has  the  form  of  a  tall  parallelogram  tipped  slightly  backwards 
and  downwards.  Its  walls  are  formed  anteriorly  by  the  lamina  ter- 
minalis, whose  upper  half  is  greatly  thickened  by  the  two  commissures, 
while  the  lower  half  remains  very  thin ;  posteriorly,  by  the  almost  ver- 
tical mammillary  body  and  tuberculum  posterius  in  line  with  the  ros- 
tral opening  of  the  aqueduct  (aq.)  and  the  thick  anterior  end  of  the 
tectum  mesencephali  (tect.}  ;  dorsally  by  the  choroid  roof  and  the 
epiphysis ;  ventrally  by  the  thin  floor  plate  containing  the  small  chiasma 
(ch.)  rostrally,  and  the  infundibular  lumen  more  caudally;  laterally, 
by  the  median  surfaces  of  the  thalami.  The  upper  half  of  the  third 
ventricle  is  almost  filled  by  the  enormous  oval  massa  intermedia,  which 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      183 

so  nearly  approaches  the  neighboring  walls  as  to  leave  on  three  sides 
of  it  only  two  very  narrow  passages  leading  into  the  aqueduct.  The 
lower  half  of  the  ventricle  is  quite  open,  and  the  thalamic  lateral  wall 
displays  a  diagonal  sulcus  running  backward  and  downward  from  the 
middle  of  the  lower  border  of  the  massa  intermedia  to  the  posterior 
wall  of  the  ventricle  just  above  the  floor.  The  ventricle  is  also  deep- 
ened laterally  just  above  the  floor  in  two  places — the  preoptic  recess 
and  the  mammillary  recess.  The  interventricular  foramen  opens  into 
the  narrow  canal  between  the  dorsal  commissure  and  the  massa  inter- 
media. 

A  very  sharp  midbrain  flexure  results  in  the  formation  of  a  deep 
and  narrow  vertical  cleft  across  the  base  of  the  brain  which  folds  a 
section  of  it  between  the  pons  and  the  mammillary  region  together  like 
the  pages  of  a  book.  The  tectum  mesencephali  (tect.)  is  entirely  con- 
cealed from  above  by  overlying  structures,  the  rostral  three-quarters  by 
the  cerebral  hemispheres  and  the  caudal  quarter  by  the  anterior  tip 
of  the  median  lobe  of  the  cerebellum.  (Figure  4  shows  a  gap  between 
the  hemisphere  and  the  cerebellum,  due  to  a  mass  of  coagulum  after- 
wards cleared  away ;  the  relations  in  Caenolestes  and  Orolestes  are  the 
same.)  "This  is  in  contrast  to  the  usual  marsupial  arrangement,  for 
the  corpora  quadrigemina  are  in  most  cases  well  exposed  dorsally. 
(Petaurus  is  another  exception;  see  Elliott  Smith,  1895,  p.  188)"  (Her- 
rick,  1921,  antea,  page  158).  The  condition  in  Orolestes  and  Caeno- 
lestes is  apparently  one  of  the  many  expressions  of  the  fore-and-aft 
compression  already  mentioned.  Partly  in  response  to  the  crumpling 
of  the  brain  base  as  described  above,  the  tectum  displays  a  marked 
caudal  prolongation,  emphasized  by  its  recurved  keel,  as  indicated  by 
the  forward  point  of  attachment  of  the  anterior  medullary  velum  (v.m. 
a.,  Fig.  10).  Laterally  the  posterior  colliculus  is  even  more  caudally 
extended  (col.  p.,  Figs.  7,  8,  10).  The  cerebellum,  which  will  be  de- 
scribed below,  fits  snugly  into  and  around  these  structures,  behind  an 
almost  perpendicular  anterior  medullary  velum,  and  below,  behind  and 
above  the  tectum. 

The  median  surface  of  the  cerebral  hemisphere  of  Orolestes  (Fig. 
5)  and  of  Caenolestes  (Fig.  6)  exhibits  a  flattened  surface  anteriorly, 
in  close  apposition  with  its  fellow,  and  a  deeply  concave  postero-median 
surface,  hollowed  out  to  accommodate  the  bulky  midbrain.  It  is  tra- 
versed by  four  prominent  arcuate  fissures,  concentrically  arranged  and 
different  in  extent  and  curvature.  These  are,  in  order  from  without 
in,  the  hippocampal,  fimbrio-dentate,  fimbrio-alvear  (to  coin  a  term), 
and  choroid  fissures. 


184  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

The  outermost  or  hippocampal  fissure  (fs.hip.,  Figs.  5  and  6)  ap- 
parently describes  almost  three-quarters  of  a  circle  in  Orolestes  (Fig. 
5),  stretching  from  the  fissura  circularis  (fs.  circ.)  upward,  backward, 
downward  and  forward  nearly  to  the  posterior  end  of  the  tuberculum 
olfactorium.  The  same  fissure  in  the  reconstruction  of  Caenolestes 
(fs.hip.,  Fig.  6)  falls  short  of  this  at  both  ends.  It  does  not  reach  the 
fissura  circularis  rostrally  and  it  ends  more  briefly  caudally  near  the 
medial  prolongation  of  the  fissura  amygdaloidea,  as  the  latter  goes 
forward  towards  the  posterior  end  of  the  choroid  fissure.  Perhaps 
also  in  Orolestes  what  appears  to  be  the  strongly  recurved  temporal 
end  of  the  hippocampal  fissure  is  really  the  fissura  amygdaloidea  me- 
dialis.  Indeed  this  interpretation  is  suggested  by  the  sudden  pro- 
nounced shallowing  and  narrowing  of  the  fissure  at  the  exact  point 
where  a  slight  diagonal  fissure  runs  into  it  from  behind.  The  pos- 
terior end  of  the  diagonal  fissure  coincides  with  the  apparent  level  of 
the  caudal  end  of  the  amygdaloid  fissure  on  the  lateral  surface,  which 
on  the  figured  lateral  view  of  the  left  hemisphere  (Fig.  3)  is  much 
less  clearly  indicated  than  on  the  right  (Fig.  5).  The  sections  of  this 
particular  right  hemisphere  will,  when  made,  clear  up  this  point,  which 
now  rests  partly  on  the  external  configuration  of  the  hemisphere  of 
Orolestes  and  partly  on  the  internal  configuration  of  that  of  Caenolestes. 

The  second  fissure,  the  fimbrio-dentate  fissure  (fs.fim.d.)  extends 
also  in  Orolestes  (Fig.  5)  to  the  fissura  circularis,  just  below  the  fis- 
sura hippocampi.  In  Caenolestes,  however,  it  drops  sharply  downward 
away  from  the  hippocampus  entirely,  and  ends  briefly  just  in  front  of 
the  dorsal  commissure  (Fig.  6).  This  portion  of  the  fissure  seems  to 
be  purely  a  response  to  the  pressure  of  a  blood  vessel  which  is  lodged 
in  the  canal  formed  by  the  corresponding  fissures  of  the  two  hemispheres 
(Figs.  6,  I7b).  The  postcommissural  and  main  portion  of  the  fimbrio- 
dentate  fissure  separates  the  gyrus  dentatus  (gy.dent.)  not  from  the 
massive  fimbria,  but  from  the  extraventricular  ammon's  horn  or  in- 
verted hippocampus,  with  its  thin  coating  of  subpial  alveus  fibers,  and 
it  is  therefore  really  an  o/wo-dentate  fissure  in  this  region.  Posteriorly 
it  is  both  less  extensive  and  less  recurved  than  the  hippocampal  fissure. 

The  third  fissure  seems  to  have  no  name,  but  it  is  nevertheless 
quite  distinct  as  the  line  of  demarcation  between  the  massive  fimbria 
and  the  alveus-coated  extraventricular  hippocampus  (ammon's  horn), 
defining  its  rolled  ventral  limit.  It  might  be  designated  as  the  fimbrio- 
alvear  fissure  (fs.fim.al.,  Figs.  6,  173;  unlabelled,  Figs.  5,  35-38).  It 
begins  rostrally  just  behind  the  dorsal  commissure  as  an  offshoot  of 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      185 

the  fimbrio-dentate  fissure,  dropping  rapidly  away  from  the  fimbrio- 
dentate  fissure  as  the  extraventricular  hippocampus  widens,  and  it 
runs  out  caudally  as  the  massive  fimbria  tapers  down  to  nothing  at  the 
posterior  end  of  the  choroid  fissure  (just  in  front  of  figure  39). 

The  innermost  or  choroid  fissure  (unlabelled,  Fig.  5;  fs.ch.,  Figs. 
6,  173,  35-39;  Fig.  39  is  really  just  behind  the  caudal  end  of  the  choroid 
fissure)  extends  from  the  ventral  tip  of  the  dorsal  commissure 
backwards  and  downwards  in  a  smooth  curve  outlining  the  ventral 
border  of  the  fimbria  and  the  thalamo-hemi spheric  junction.  In  Caeno- 
lestes,  as  noted  above,  the  internal  medial  prolongation  of  the  fissura 
amygdaloidea  can  be  quite  clearly  traced  forward  to  the  neighborhood 
of  the  caudal  end  of  the  choroid  fissure. 

Other  features  of  the  medial  surface  of  the  hemisphere  are  the 
tuberculum  olfactorium  in  profile  behind  the  olfactory  bulb,  followed 
immediately  in  Orolestes  by  the  small  ventro-median  eminence  (Fig.  5, 
unlabelled)  which  is  probably  due  to  the  nucleus  of  the  lateral  ol- 
factory tract  (nuc.tr.ol.l.,  Figs.  36-38).  This  small  tubercle  appears 
in  Notoryctes  (Elliot  Smith,  i895b)  and  in  a  number  of  other  mar- 
supials and  lowly  eutherian  mammals,  but  is  apparently  absent  or 
not  very  prominent  in  Caenolestes,  although  the  nucleus,  as  will  appear 
below,  is  well  developed.  It  is,  however,  partly  covered  by  the  tuber- 
culum olfactorium,  and  this  masks  it  to  some  extent.  The  amygdaloid 
complex  (amg.,  Fig.  6;  unlabelled,  Fig.  5)  extends  upward  as  high 
as  the  position  of  the  medial  portion  of  the  amygdaloid  fissure  (fs.amg. 
w.)  already  described,  and  the  posterior  pyriform  cortex  (cx.pir.p.) 
curves  around  the  caudal  pole  of  the  hemisphere  to  form  the  more 
temporal  subicular  border  between  the  medial  extensions  of  the  rhinal 
and  amygdaloid  fissures,  in  the  familiar  mammalian  pattern.  From 
the  postero-medial  extension  of  the  rhinal  fissure  (fs.rh.m.)  forward 
to  the  antero-medial  extension  of  the  same  fissure  (fs.rh.a.)  the  space 
above  the  hippocampal  fissure  is  occupied  by  neopallium.  In  front 
of  the  anterior  rhinal  fissure  the  hippocampal  cortex  is  in  cellular  con- 
tinuity with  the  dorso-medial  portion  of  the  anterior  olfactory  nucleus 
(nuc.  ol.  ant.d.,  Fig.  26),  the  peduncular  or  postbulbar  gray  matter 
which  laterally  merges  without  interruption  into  the  pyriform  cortex 
(see  page  192).  Save  for  these  rostral  and  caudal  junctions  the  lateral 
and  medial  olfactory  cortices  are  split  apart  dorsally  by  the  wedge- 
like  neopallial  cap  of  the  hemisphere. 

The  entire  length  of  the  fissura  prima  of  His  appears  upon  the 
median  surface  of  the  hemisphere  as  the  visible  portions  of  the  fis- 
sura circularis  rostrally  and  fissura  rhinalis  arcuata  caudally  (see  page 


i86  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

178).  Between  it  and  the  lamina  terminalis  lies  the  precommissural 
area  (paraterminal  body)  of  Elliot  Smith,  the  parolfactory  region  of 
Johnston,  the  septum  of  most  neurologists  (a.  prcom.,  Fig.  6;  Unla- 
belled,  Fig.  5).  The  nucleus  parolfactorius  medialis,  the  nucleus  of 
the  diagonal  band  of  Broca,  the  precommissural  fornix,  olfacto-hippo- 
campal  fibers,  and  the  medial  forebrain  bundle  (fasciculus  medialis 
telencephali),  are  the  more  superficial  structures  lying  above  the  pos- 
terior part  of  the  fissura  prima  (medial  part  of  the  fissura  rhinalis 
arcuata).  The  sharply  grooved  dorso-medial  margin  of  the  hemisphere 
is  the  imprint  of  the  longitudinal  sinus,  which  is  seen  in  position  in  fig- 
ure 4. 

The  enormously  expanded  lateral  ventricle  of  Caenolestes  (v.l., 
Figs.  28-42)  is  probably  not  pathological,  but  the  result  of  defective 
fixation.  As  a  cursory  inspection  of  the  more  caudal  sections  will 
show,  a  recurved  temporal  horn  has  not  even  begun  to  form.  The 
posterior  horn,  present  in  the  Virginia  opossum,  is  also  absent  in 
Caenolestes.  In  the  lower  rostral  wall  of  the  ventricle  a  flaring  opening 
leads  into  a  narrow  crooked  canal,  which  in  turn  expands  into  a  widened 
terminal  sac.  These  constitute  the  olfactory  ventricle  (v.ol.,  Figs.  12- 
15,  17,23-27). 

Cerebellum. — (Figs.  I,  3,  4,  7-10,  ica  and  lob,  41-44;  also  Herrick, 
1921,  antea,  pi.  XXI.)  The  cerebellum  of  Caenolestes  and  Orolestes 
(the  two  forms  are  practically  identical  in  general  structure)  very 
neatly  fills  the  gap  between  those  of  Notoryctes  and  Perameles,  the  two 
simplest  mammalian  cerebella  hitherto  described.  (Elliot  Smith,  i9O2b, 
I9O3C,  d,  e).  The  cerebella  of  the  insectivores  Macroscelides  and 
Erinaceus  (also  described  by  Elliott  Smith,  I9O2C,  i9O3c)  probably  oc- 
cupy the  fourth  and  fifth  places  in  the  series.  The  cerebellum  of  the 
Virginia  opossum  is  considerally  more  complex  than  any  of  those 
mentioned. 

Of  the  three  fundamental  cerebellar  lobes  separated  by  the  fissura 
prima  (fs.pr.)  and  fissura  secunda  (fs.s.),  the  median  and  posterior 
lobes  differ  little  in  the  first  four  forms  named  above.  But  the  anterior 
lobe  grades  very  clearly  from  the  simple  unfissured  lobe  of  Notorcytes 
(loa),  through  Caenolestes  (Figs.  4,  7-10)  or  Orolestes,  with  two 
lobules  separated  by  the  deep  fissura  preculminata  (fs.prcul.),  and  a 
third  well  developed  lobulus,  the  lingula  (/#.),  which  is  not  separated 
from  the  rest  by  a  fissure,  to  Perameles  (Fig.  lob),  with  an  anterior 
lobe  crossed  in  the  midline  by  four  fissures  of  varying  depth.  The 
anterior  lobe  and  the  fissura  prima  are  entirely  concealed  in  the  intact 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      187 

brain.  The  visible  portion  of  the  cerebellum  is  formed  by  the  exposed 
parts  of  the  median  and  posterior  lobes.  It  is  crossed  in  the  midline  by  a 
ventrally  concave  fissure,  the  fissura  secunda  (/J.J.),  separating  a  small 
postero-medial  convolution,  the  uvula  (uv.)  belonging  to  the  posterior 
lobe,  from  the  larger  median  lobe  above.  In  Caenolestes  but  not  in 
Orolestes,  the  secondary  fissura  suprapyramidalis  (fs.spyr.),  shallow 
but  sharp,  divides  the  median  lobe  into  a  large  suprapyramidal  region 
(p.spyr.)  and  a  slender  curved  ventral  convolution,  the  pyramis  (pyr.), 
concentric  with  the  uvula,  which  fills  the  ventral  concavity  of  the  fissura 
secunda.  The  other  lobule  of  the  posterior  lobe,  the  nodulus  (nod.), 
lies  entirely  hidden  on  the  ventral  surface  of  the  cerebellum,  being 
separated  from  the  uvula  by  the  concealed  fissura  postnodularis  (fs. 
pnod.}.  The  cerebellar  ventricle  (v.cb.)  is  seen  between  the  nodulus 
and  the  lingula. 

The  large  mushroom-shaped  pedunculate  lateral  masses  projecting 
beyond  the  lateral  lobes  (these  indicated  only  by  a  very  slight  partial 
constriction)  are  the  paraflocculi  (pfloc.),  the  dorsal  components  of 
the  floccular  lobes.  The  ventral  components  are  the  tiny  flaplike 
flocculi,  concealed  from  view,  being  plastered  against  the  medulla 
beneath  the  paraflocculi,  from  which  they  are  separated  by  the  fissura 
floccularis  (fs.floc.).  The  fissura  parafloccularis  (fs. pfloc.),  visible 
laterally,  separates  the  paraflocculus  from  the  median  lobe.  It  cuts 
down  to  the  floccular  peduncle  between  these  structures. 

All  the  cerebellum  save  the  floccular  lobe  and  the  lateral  lobes  (area 
pteroidea,  a.pt.,  in  part)  corresponds  probably  to  the  vermis  of  higher 
cerebella.  The  lateral  lobes  similarly  are  probably  homologous  with 
the  cerebellar  hemispheres. 

The  deep  nuclei  (deep  nuc.)  form  a  pair  of  large  oval  masses  almost 
meeting  in  the  midline,  with  a  large  area  of  their  ventral  surface  ex- 
posed in  the  roof  of  the  fourth  ventricle.  They  exhibit  the  mammalian 
characteristic  of  complete  separation  from  Deiters'  nucleus,  as  van 
Hoevell  (1916)  found  in  some  other  marsupials.  There  are  only 
slight  indications  of  differentiation  into  separate  nuclei  (dentate  and 
roof,  nuclei). 

Since  the  cerebellum  of  Caenolestes  and  Orolestes  will  form  part 
of  the  next  report  on  these  brains,  it  need  not  be  further  discussed  at 
this  time.  The  intra-  and  extracerebellar  relations  will  be  partially 
clarified  by  the  diagrammatic  reconstructions,  figures  7-10,  together 
with  the  sections,  figures  41-44.  The  nomenclature  follows  the  usage 
of  Elliot  Smith. 


i88  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

Measurements. — The  dimensions  of  these  three  brains  as  meas- 
ured on  the  alcoholic  specimens  are  as  follows: 

C.obs.*  O.inca 

18507  194948       194921 

F.M.  U.S.N.M.  U.S.N.M. 

1.  Total  length,   tip  of   olfactory   bulb   to   first 

spinal    nerve    14.1  mm.     16.5  mm.     19.1  mm. 

2.  Length,    tip    of    olfactory    bulb    to    end    of 

cerebral  hemisphere    2.6  3.0  3.6 

3.  Length  of   cerebral  hemisphere 10.0  9.3  8.8 

4.  Length  of  cerebellum  on  longitudinal  axis  of 

brain  in  median  plane    3.0  5.0  6.2 

5.  Greatest  width  of   olfactory  bulbs 7.6  7.8  7.9 

6.  Greatest  width  of  both  cerebral  hemispheres  n.8  n.8  12.6 

7.  Total  width  of  cerebellum  and  floccular  lobes  n.o  11.7  — 

8.  Width  of   cerebellum  exclusive  of   floccular 

lobes    8.8  i  i.o  i  i.o 

9.  Maximum  vertical  height  of  cerebral  hemi- 
spheres   7-9  8.9 

10.  Distance    of    orbital    fissure    behind    rostral 

tip  of  hemisphere (2.0)  *  2.3*  2.4 

11.  Distance  of   rhinal  fissure   below   vertex  at 

orbital   fissure    2.7*  4.2 

12.  Length,  head  and  body   107.0  (89.0)  ( 102.0) 

13.  Length,    tail    118.0  115.0  116.0 

14.  Length,    total    225.0  204.0  218.0 

15.  Weight   of  brain,   immediately   out  of   80% 

alcohol    760.9  mg.  961.0  mg. 

*  C.    obscurus,    measurements    i-n,    from    Herrick    (1921) ;    12-14,    from    Dr. 

Osgood. 

*  Position  of  orbital  fissure  in   C.  obscurus  is  "about  one-fifth  of  the  distance 

backward  from  the  frontal  to  the  posterior  pole  of  the  hemisphere". 

*  Orbital  fissure  not  very  clear  in  O.  inca,  No.  194948. 

O.  inca  No.  194921  had  probably  all  of  cervical  cord  attached,  but  lost  parafloc- 

cular  lobes  on  removal  from  the  skull. 
O.  inca  No.  194948  was  broken  off  just  behind  cerebellum,  in  removing  it  from 

the  skull. 

INTERNAL  ANATOMY 

The  twenty-two  simplified  cross  sections  from  the  hemisphere  of 
Caenolestes  obscurus  (Figs.  23-44)  and  the  linear  reconstructions  of 
the  hemisphere  or  of  parts  of  it  (Figs.  12-15,  J7a  and  b)  may  perhaps, 
with  the  aid  of  the  figures  and  descriptions  of  the  external  surfaces, 
render  a  brief  and  incomplete  account  of  the  internal  anatomy  com- 
prehensible. The  following  table  of  critical  levels  and  their  section 
numbers  may  also  assist  in  the  orientation  of  internal  structures. 
Sec.  i — Anterior  tip  of  the  olfactory  bulb. 

Sec.  225 — Anterior  tip  of  the  frontal  lobe  of  the  neopallium  (Fig.  23). 
Sec.  400 — Caudal  (medio-ventral)  limit  of  the  olfactory  bulb,  adjoin- 
ing the  tuberculum  olfactorium  (Fig.  25). 
Sec.  710— Caudal  limit  of  the  tuberculum,  adjoining  the  amygdaloid 

complex  (Fig.  36). 
Sec.  935 — Posterior  tip  of  the  cerebral  hemisphere  (Fig.  44). 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      189 

PRIMARY  OLFACTORY  AREA 

The  description  of  the  histological  pattern  of  the  bulbar  formation, 
both  ordinary  and  accessory,  may  be  dispensed  with  here,  in  view  of 
its  similarity  to  that  of  the  Virginia  opossum,  already  described  in 
detail  by  McCotter  (1912).  Contrary  to  the  condition  in  the  opossum, 
however,  the  horizontal  diameter  of  the  olfactory  bulb  in  Caenolestes 
is  greater  than  the  vertical  diameter.  The  usual  medial  displacement 
of  the  olfactory  ventricle  is  emphasized  here  by  the  considerable  dorso- 
ventral  flattening  of  the  olfactory  bulb.  This  displacement  may  per- 
haps be  a  condition  of  interest  in  connection  with  the  disposition  of  the 
olfactory  tract  fibers. 

The  well  developed  accessory  olfactory  bulb  (b.ol.ac.,  Figs.  12,  14, 
23-24)  is  embedded  in  the  postero-dorsal  olfactory  formation  beneath 
the  overhanging  frontal  pole  of  the  neopallium.  It  is  outlined  by  a 
slight  fissure.  Its  peripheral  nerve,  the  vomeronasal  nerve  (n.vn., 
Figs.  12,  14,  23-24)  is  quite  clear  as  it  curves  up  over  the  medio-dorsal 
angle  of  the  olfactory  bulb  and  spreads  out  to  cover  the  surface  of  the 
accessory  bulb.  The  secondary  tract  of  the  accessory  bulb  passes  lat- 
erally and  superficially  and  helps  to  make  up  the  pars  dorsalis  of  the 
lateral  olfactory  tract  (tr.ol.l.d.,  Figs.  23  ff.)  The  olfactory  tracts, 
which,  as  Cajal  pointed  out  (1911),  arise  indiscriminately  from  all 
parts  of  the  olfactory  formation  and  are  without  specificity  (save  for 
the  fibers  from  the  accessory  bulb),  first  condense  in  the  center  of 
the  bulb  some  distance  in  front  of  the  olfactory  ventricle  (tr.ol.,  Figs. 
12-15).  Since  they  so  quickly  begin  to  distribute  to  the  secondary 
olfactory  areas  and  even  in  part  (intermediate  olfactory  tract)  to  re- 
ceive fibers  from  them,  their  description  will  be  continued  below  Under 
that  head. 

SECONDARY  OLFACTORY  AREAS 

The  olfactory  fibers  spin  a  whorl  around  the  olfactory  ventricle, 
much  thicker  on  the  lateral  side  than  elsewhere.  A  second  independent 
half-whorl  forms  within  the  first,  between  its  thick  lateral  portion  and 
the  ventricle.  This  mass  of  fibers,  composed  of  secondary  (direct)  and 
tertiary  olfactory  fibers,  soon  rounds  up  in  the  lateral  wall  of  the 
ventricle  as  the  intermediate  olfactory  tract  (tr.ol.i.,  Figs.  23-28), 
largely  forming  the  rostral  limb  of  the  anterior  commissure  (c.a.,  Figs. 
24-28).  Meanwhile,  between  it  and  the  lateral  part  of  the  first  whorl 
(the  massive  portion  of  the  lateral  olfactory  tract,  tr.ol.i.)  appear  the 
most  anterior  cells  of  the  peduncular  grey  or  anterior  olfactory  nucleus, 
pars  lateralis  (nuc.ol.ant.L,  Figs.  12-15,  17,  23-25).  This  growing 


190    FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

cell  mass  rapidly  increases  the  widening  interval  between  the  lateral 
part  of  the  big  whorl  externally  and  the  half -whorl  internally,  and 
immediately  begins  to  contribute  tertiary  olfactory  fibers  to  the  inner 
half-whorl,  the  intermediate  or  commissural  olfactory  tract,  which  also 
continues  to  receive  secondary  olfactory  fibers  from  all  parts  of  the 
olfactory  formation.  Still  farther  back  the  tuberculum  olfactorium 
(t.ol.,  Figs.  13,  28)  cuts  the  outer  whorl  into  two  parts,  the  lateral 
olfactory  tract,  much  the  largest  of  all  the  olfactory  tracts,  and  the 
slender  medial  olfactory  tract  (tr.ol.m.,  Fig.  28).  The  intermediate  tract, 
which  retains  its  subependymal  position  back  to  the  point  where  the 
head  of  the  caudate  nucleus  (nuc.  caud.,  Fig.  28)  overlies  it,  soon  con- 
tains perhaps  more  tertiary  than  secondary  fibers,  and  is  therefore 
labelled  only  anterior  commissure  (c.a.)  from  figure  24  back. 

The  lateral  olfactory  tract,  the  most  widely  distributed  of  the  three, 
becomes  superficial  (tr.ol.l.,  Figs.  3,  12,  25)  behind  the  fissura  circu- 
laris,  forming  the  medullated  external  fiber  layer  of  the  lateral  sur- 
face of  the  hemisphere  over  most  of  its  extent  below  the  rhinal  fissure. 
It  may  be  divided  into  several  parts :  anteriorly,  two  peduncles,  a  sub- 
ventricular  ventral  peduncle  (tr.ol.l.p.v.,  Figs.  24-27)  a  part  of  which 
caudally  becomes  independent  and  is  then  called  the  medial  olfactory 
tract  {tr.ol.m.,  Fig.  28)  as  noted  above,  and  a  supraventricular  dorsal 
peduncle  (tr.oLl.p.d.,  Figs.  23-24)  with  two  roots,  a  ventral  root,  below 
the  accessory  olfactory  bulb,  and  a  dorsal  superficial  root  which  is  con- 
tinued caudally  as  the  pars  dorsalis  of  the  lateral  olfactory  tract  (tr.ol.- 
l.d.,  Figs  25  ff.)  ;  a  clubshaped  massive  portion  (tr.ol.l.,  Figs.  23-35) 
filling  the  interval  between  the  endorhinal  and  the  rhinal  arcuate  fis- 
sures, wide  anteriorly  but  decreasing  to  the  vanishing  point  as  these 
fissures  meet  (fs.erh.,  fs.rh.arc.,  Figs.  13,  33)  ;  posteriorly  a  pars 
ventralis  (tr.ol.l.v.,  Figs.  29  ff.),  distributing  to  the  tuberculum,  nucleus 
of  the  lateral  olfactory  tract  (nuc. tr.ol.l.,  Figs.  12,  36-38)  and  other 
amygdaloid  nuclei,  and  perhaps  also  to  the  diagonal  band  nucleus 
(nuc.d.b.,  Figs.  32-33).  The  small  medial  olfactory  tract  (tr.ol.m., 
Fig.  28)  can  be  seen  turning  sharply  downward  into  the  plexiform  layer 
of  the  tuberculum  at  its  medial  border,  where  some  observers  (e.g., 
Livini,  1908)  recognize  a  terminal  nucleus  of  the  median  olfactory 
tract  in  a  large  median  rolled  portion  of  the  tubercular  formation.  Such 
a  structure  is  present  here  (nuc.ol.m.,  Fig.  31).  Beccari  (1910) 
inclines  to  doubt  the  distribution  of  secondary  olfactory  fibers  to  any 
extent  to  the  tuberculum,  but  such  evidence  as  these  sections  give, 
while  not  at  all  conclusive,  seems  to  point  to  it. 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      191 

Anterior  olfactory  nucleus. — The  less  differentiated  postbulbar  gray 
matter  of  the  rhinencephalon  has  been  termed  by  Herrick  (1910, 
pp.  191-2,  Figs.  9  and  10)  the  anterior  olfactory  nucleus.  In  Caeno- 
lestes  it  is  almost  entirely  intrabulbar  and  in  direct  continuity  with 
the  rest  of  the  more  ventral  gray  subjacent  to  the  lateral  olfactory  tract. 
Rostrally  therefore  it  completely  surrounds  the  olfactory  ventricle.  It 
corresponds  essentially  to  the  peduncular  gray  of  Cajal,  and  is  not 
included  in  his  pyriform  lobe.  It  is  here  excluded  from  the  pyriform 
lobe,  though  earlier,  following  Johnston's  definition  of  the  pyriform 
lobe  as  all  the  gray  underlying  the  lateral  olfactory  tract,  I  counted  it 
a  part  of  the  pyriform  lobe  (Obenchain,  1923).  Since  its  lateral 
border  is  farthest  advanced  rostrally,  this  peri  ventricular  rhinencephalic 
ring  appears  incomplete  in  the  more  rostral  sections  (Figs.  23-26),  and 
since  the  pyriform  cortex  is  also  advanced  rostrally  beyond  some  por- 
tions of  the  anterior  olfactory  nucleus,  the  whole  of  the  latter  is  never 
seen  in  any  transverse  section.  The  anterior  olfactory  nucleus,  on  both 
topographical  and  histological  grounds,  may  be  subdivided  into  several 
parts,  all  in  cellular  continuity  rostrally.  I  have,  as  far  as  seemed 
advisable,  conformed  to  the  terminology  applied  to  the  different  parts 
of  the  anterior  olfactory  nucleus  in  the  Virginia  opossum  (Herrick, 
19243).  Divergences  will  be  noted  as  they  occur. 

The  lateral  part  (nuc.ol.ant.L,  Figs.  12-15,  I7>  23~25)>  rostrally 
most  advanced,  passes  caudally  without  interruption  into  true  pyriform 
cortex,  and  therefore  has  no  really  definite  posterior  limit.  The  pars 
lateralis  is  extended  medially  above  the  olfactory  ventricle  as  the  pars 
dorsalis  (nuc.ol.ant.d.,  Figs.  12-15,  I7>  25~26) ;  it  corresponds  to 
Cajal's  superior  peduncular  nucleus.  Just  in  front  of  the  antero- 
median  extension  of  the  rhinal  fissure  (fs.rh.a.)  it  comes  to  the  sur- 
face, as  noted  above,  and  just  behind  it  fuses  with  the  cell  mass  of  the 
overlying  neopallial  frontal  pole.  More  caudally  and  below  the  ventricle 
the  anterior  olfactory  nucleus  is  also  prolonged  medially  by  another 
hooklike  extension,  the  pars  later o-ventralis  (nuc.ol.ant.l.v.,  Figs.  12,  15, 
17,  26),  and  beyond  that  the  pars  posterior  (nuc.ol.ant.p.,  Figs.  12,  15, 
17,  27-29),  which  fills  the  wedgelike  interval  in  front  of  the  fusion  of 
the  head  of  the  caudate  nucleus  and  the  tuberculum  olf actor ium.  The 
pars  posterior  extends  further  back  than  other  portion  of  the  anterior 
olfactory  nucleus, 'but  it  is  neither  in  continuity  with  the  caudate  nucleus 
— forming  the  rostral  portion  of  its  head,  as  in  the  turtle,  Cistudo 
Carolina  (Johnston,  1915) — nor  with  the  deeper  layer  of  the  tuber- 
culum, as  in  the  alligator  (Crosby,  1917).  It  stops  bluntly  just  ros- 
tral to  the  fusion  of  the  caudate  head  and  the  tuberculum.  This  part 


192  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

of  the  anterior  olfactory  nucleus  never  shows  any  appreciable  degree 
of  differentiation,  probably  owing  to  the  too  purely  olfactory  character 
of  its  connections.  The  latero-ventral  and  posterior  parts  of  the  an- 
terior olfactory  nucleus  as  here  described  do  not  exactly  coincide  with 
the  divisions  in  the  Virginia  opossum.  More  caudally  the  dorsal  and 
posterior  parts  meet  medially,  thus  completing  the  anterior  periventric- 
ular  rhinencephalic  ring  by  means  of  a  pars  medialis  (nuc.ol.ant.m., 
Figs.  12,  15,  17,  26). 

At  the  dorso-medio-rostral  border  of  this  ring  there  is  a  small 
superficial  condensation  of  cells.  This  is  the  anterior  tip  of  the 
hippocampal  formation  (hip. a.,  Figs.  12,  17;  ex. hip. a.,  Fig.  26).  It 
corresponds  to  a  similar  dorso-lateral  pyriform  condensation  (cx.pir.a., 
Fig.  25),  which,  although  it  appears  in  more  rostral  sections,  does  not 
quite  reach  the  rostral  border  of  the  ring.  The  lateral  and  medial 
olfactory  cortices  (pyriform  and  hippocampal)  are  thus  only  indirectly 
continuous  rostrally  by  a  double  bond  (supra-  and  infra  ventricular) 
through  the  agency  of  the  anterior  olfactory  nucleus,  while,  as  we  have 
seen,  they  are  directly  continuous  caudally.  There  are  thus  two  peri- 
ventricular  rhinencephalic  rings,  a  smaller  transverse  and  a  larger  hori- 
zontal one,  which  are  partially  fused  anteriorly. 

The  medial  part  of  the  anterior  olfactory  nucleus  soon  passes  ob- 
scurely backward  into  the  lateral  parolfactory  nucleus  (nuc.pol.L,  at  a 
level  between  figures  26  and  27).  The  latero-ventral  part,  which  is 
transitional  between  the  lateral  and  posterior  parts,  continues  back- 
ward into  the  ventral  pyriform  cortex  underlying  the  massive  portion 
of  the  lateral  olfactory  tract  along  the  lateral  border  of  the  tuberculum 
(pars  ventralis,  lobus  piriformis,  Gray,  1924;  ex.pir.v.,  Figs.  27-33), 
and  still  more  caudally  perhaps  into  the  diffuse  region  of  the  anterior 
perforated  space  (Johnston,  1923)  (l.perf.a.,  Figs.  35-38),  along 
the  anterior  portion  of  the  amygdaloid  fissure  (fs.atmg.,  Figs.  12,  15, 
36-38). 

One  other — and  the  most  interesting — part  of  the  anterior  olfactory 
nucleus  remains  to  be  described,  the  pars  externa  (nuc.ol.ant. ex.,  Figs. 
13,  15,  24-26).  It  is  apparently  the  result  of  the  doubling  of  the  in- 
trabulbar  portion  of  the  lateral  part  of  the  anterior  olfactory  nucleus, 
which  is  the  only  portion  to  exhibit  this  phenomenon.  The  external 
nucleus  appears  rostrally  as  a  vertical  plate  of  smaller  cells  between 
the  pars  lateralis  and  the  massive  lateral  olfactory  tract.  It  is  separated 
from  the  subjacent  pars  lateralis  by  a  wide  plexiform  layer  narrowing 
ventrally  to  suggest  cellular  continuity  between  the  two  nuclei.  Ros- 
trally a  very  narrow  but  definite  external  plexiform  layer  separates  it 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      193 

from  the  fibers  of  the  lateral  olfactory  tract.  Caudally  the  external 
nucleus,  or  external  part  of  the  lateral  part  of  the  anterior  olfactory 
nucleus  (to  give  it  its  full  designation)  slides  downward  along  the 
internal  surface  of  the  lateral  olfactory  tract,  gradually  diminishing 
and  rounding  up  into  a  small  mass  of  cells  which,  as  it  slips  medial  ward 
around  the  inside  of  the  "elbow"  of  the  lateral  tract,  becomes  counter- 
sunk in  a  space  due  to  the  clearing  away  of  some  of  the  blue  tract 
fibers.  It  is  here  opposite  to  the  pars  latero-ventralis,  but  its  main 
bulk  lies  opposite  the  pars  lateralis — there  is  no  definite  boundary  be- 
tween these  two  parts.  The  general  form  of  the  external  nucleus  is 
that  of  a  long  and  slender  pennant  (Fig.  13)  whose  caudo-ventrally 
extended  tail  curves  far  medialward  to  end  almost  beneath  the  olfac- 
tory ventricle  (Fig.  15).  The  external  nucleus  of  Caenolestes  is  thus 
entirely  intrabulbar,  so  that  it  faces  the  granular  olfactory  formation 
across  the  olfactory  tract.  Dr.  Herrick  finds  that  the  Virginia  opos- 
sum also  possesses  an  essentially  similar  nucleus,  save  that  it  lacks  an 
external  plexiform  layer,  and  is  caudally  unevenly  swallow-tailed.  In 
Caenolestes  a  small  dorsal  group  of  cells  tends  to  be  separated  from 
the  rest  by  the  fibers  of  the  ventral  root  of  the  dorsal  peduncle  of  the 
lateral  olfactory  tract.  Rothig  (1910,  Fig.  i)  figures  but  does  not 
name  a  small  vertical  cell  plate  in  Didelphis  marsupialis,  which  is 
plastered  against  the  inner  aspect  of  the  dorsal  thin  part  of  the  lateral 
olfactory  tract  behind  the  bulbar  formation,  a'  dorsal  shift  which  would 
bring  it  nearer  to  the  remaining  bulbar  formation  at  this  level.  The 
entire  width  of  the  plexiform  layer  separates  it  from  the  underlying 
pyriform  cortex.  As  this  is  the  most  anterior  section  figured  by  Rothig, 
the  more  rostral  extent  and  relations  of  this  cell  plate  are  unknown  to 
me.  Livini  (1908,  Fig.  2)  shows  in  Hypsiprymnus  rufescens  a  doub- 
ling of  the  intrabulbar  portion  of  his  anterior  pyriform  lobe  (which  cor- 
responds to  the  lateral  part  of  the  anterior  olfactory  nucleus  as  de- 
scribed here)  into  two  nearly  equal  vertical  plates  of  cells  only  narrowly 
separated,  the  outer  of  which  seems  to  be  composed  of  somewhat  smaller 
cells.  In  a  Nissl  series  of  a  white  rat  brain  I  find  the  anterior  olfac- 
tory nucleus  also  doubling  laterally  in  the  olfactory  peduncle,  in  the  gap 
between  the  dorsal  and  ventral  edges  of  the  bulbar  formation,  which 
in  cross  section  has  just  broken  in  two — in  other  words,  just  behind 
the  fissura  circularis.  The  external  portion  is  a  more  condensed  ver- 
tical plate  of  cells,  immediately  beneath  the  massive  lateral  olfactory 
tract,  bridging  the  gap  in  the  olfactory  formation  and  therefore  in 
contact  with  it  at  its  dorsal  and  ventral  margins.  More  caudally,  with 
the  widening  of  the  gap  between  the  receding  edges  of  the  olfactory 


194  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

formation,  the  external  nucleus  separates  into  two  parts,  dorsal  and 
ventral,  which  diminish  as  they  separate  more  and  more,  and  finally 
end  as  tiny  cell  masses  at  the  upper  and  lower  borders  of  the  bulbar 
formation  near  its  caudal  limits.  Its  general  form  is  that  of,  an  evenly 
swallow-tailed  pennant.  The  Winkler-Potter  rabbit  and  cat  atlases 
(1911,  1914)  give  no  figures  showing  such  a  nucleus.  Cajal  (1911) 
neither  mentions  nor  figures  it,  a  circumstance  which  is  perhaps  to 
be  ascribed  to  its  failure  to  impregnate  in  his  rich  collection  of  Golgi 
series  of  the  brain  of  the  mouse.  His  only  figure  showing  the  lateral 
peduncular  gray  (1911,  II,  Fig.  431,  p.  696,  mouse,  5  days  old,  after 
Calleja)  is  apparently  a  horizontal  section,  which  may  miss  the  level 
of  the  external  nucleus,  if  there  be  such  a  nucleus  in  the  mouse.  I 
have  so  far  found  no  reference  in  the  literature  to  this  curious  cell 
mass  beyond  the  Rothig  and  Livini  figures  cited  above,  in  which  it 
was  unlabelled. 

The  interpretation  of  this  nucleus  offers  apparently  no  great  diffi- 
culty. It  is,  probably,  some  sort  of  reenforcing  or  stepping-up  device 
for  olfactory  stimuli,  an  accessory  olfactory  nucleus,  discharging  its 
afferent  fibers  into  the  subjacent  lateral  part  of  the  anterior  olfactory 
nucleus.  It  is  a  regulatory  response  probably  provoked  in  part  by 
the  antero-posterior  compression  of  this  highly  macrosmatic  type 
of  brain  at  the  point  where  the  neurobiotactic  attraction  of  accumulated 
secondary  fibers  is  strongest.  Does  this  nucleus  represent  the  morpho- 
logical anterior  end  of  the  anterior  olfactory  nucleus,  detached  and  car- 
ried back  by  the  more  anterior  collaterals  of  the  lateral  olfactory  tract, 
either  actually  or  by  being  held  fast  by  them  during  the  progress  of  the 
compression  which  telescopes  bulb  and  anterior  end  of  the  extrabulbar 
rhinencephalon  ?  Or  is  it  delaminated  in  situ  from  the  subjacent  cell 
mass,  being  composed  of  the  cells  which  possess  no  basilar  dendrites 
and  are  therefore  unable  to  resist  the  unopposed  neurobiotactic  influence 
of  the  overlying  fibers?  It  is  not  inconceivable  that  both  delamination 
and  dislocation  may  have  operated  in  its  formation.  Although  Cajal 
neither  pictures  nor  describes  the  external  nucleus,  and  gives  but  one 
copied  figure  of  the  lateral  peduncular  formation  in  connection  with 
which  it  arises,  his  figure  and  description  of  the  region  immediately 
behind  its  due  position  furnish  apparently  unmistakable  clues,  in  the 
light  of  Kappers'  concept  of  neurobiotaxis,  of  the  mode  and  causes  of 
its  formation  further  forward  (1911,  II.,  Fig.  433,  p.  680,  rabbit,  aged 
25  days).  In  the  cortex  of  his  "frontal  lobe"  (anterior  area  of  pyri- 
form  cortex,  Gray,  1924)  the  third  layer  (outer  cell  layer,  superficial 
polymorphs  or  medium  pyramids)  contains  cells  without  basilar  den- 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      195 

drites,  all  their  dendrites  being  directed  outward  toward  the  fibrillar  layer 
(lateral  olfactory  tract).  These  cells  form,  according  to  Cajal,  a 
rather  precise  wavy  band,  not  however  separated  off  from  the  cells 
below,  and  they  are  extremely  variable  in  form.  The  deeper  pyramidal 
cells  and  the  still  deeper  polymorphic  cells  are  provided  with  both 
ascending  and  descending  dendrites,  and  are  therefore  doubly  anchored 
and  not  subject  to  extreme  outward  displacement  by  unbalanced  olfac- 
tory stimuli  of  the  lateral  olfactory  tract,  as  are  the  more  superficial 
cells  without  descending  dendrites.  The  balancing  attraction  resides, 
probably,  in  the  heterolateral  secondary  and  perhaps  in  the  tertiary 
fibers  in  the  rostral  limb  of  the  anterior  commissure,  which  lies  beneath 
the  lateral  portion  of  the  anterior  olfactory  nucleus.  Whether  any  other 
fibers  reach  it  from  this  direction,  as,  for  example,  homolateral  second- 
ary fibers  of  the  intermediate  olfactory  tract,  I  do  not  know.  At  any 
rate  this  external  part  of  the  anterior  olfactory  nucleus  is  slung  like  a 
hammock  between  two  opposing  neurobiotactic  forces,  and  the  doubling 
or  splitting  of  the  nucleus  in  this  region  expresses  the  resolution  of  the 
situation.  The  deep  and  main  portion  of  the  nucleus  in  this  region 
lies  in  Caenolestes  closer  to  the  anterior  commissure  than  to  the  lateral 
olfactory  tract.  Cajal  (1911,  II.,  p.  678)  says  that  the  cortex  of  the 
olfactory  peduncle  and  of  the  "frontal  lobe"  (which  lies  next  behind 
it)  are  essentially  the  same  in  structure.  We  should  indeed  expect  to 
find  the  lateral  peduncular  gray  less  differentiated,  with  perhaps  shorter 
axons  and  fewer  descending  dendrites,  and  with  only  a  slight  tendency 
toward  cortical  lamination.  If  the  situation  is  as  sketched  we  have  a 
clear  and  exquisite  illustration  of  the  two-fold  activity  of  neurobiotaxis 
at  work.  Since  the  lateral  olfactory  tract  is  always  in  macrosmatic  ani- 
mals an  exceedingly  heavy  mass  of  fibers  it  is  not  improbable  that  the 
external  olfactory  nucleus  is  well  developed  in  at  least  all  those  forms 
whose  olfactory  peduncle  (anterior  olfactory  nucleus)  is  jammed  for- 
ward and  largely  enclosed  within  the  bulbar  formation,  as  in  the  forms 
mentioned  here.  We  should  expect  to  find  vestigial  traces  of  it  wide- 
spread among  mammals  in  general,  and  it  has  most  likely  already  found 
its  way  into  the  literature  in  some  form  or  other. 

Tuberculum  olfactorium. — The  enormous  tuberculum  olfactorium 
begins  in  these  sections  latero-ventrally  (t.ol.  (i.C.),  Figs.  2-5,  12,  13, 
15,  28-36),  instead  of  medio-ventrally  as  in  the  Virginia  opossum.  It 
rapidly  expands  medialward  to  occupy  the  entire  width  of  the  base  of 
the  brain,  and  caudally  to  a  point  beyond  the  middle  of  the  hemisphere 
(section  710  in  a  hemisphere  numbering  935  sections).  Antero-medially 
it  turns  up  on  the  medial  surface  for  a  short  distance,  where  it  is  de- 


196  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

limited  by  the  medial  portion  of  the  fissura  rhinalis  arcuta  (fs.rh.arc.}, 
which  does  not  as  here  described  quite  agree  with  Beccari's  account 
( 1910,  Fig.  18) ;  he  extends  the  anterior  and  posterior  parts  of  the 
encircling  fissure  of  the  tuberculum  upward  in  the  medial  wall  to  meet 
at  the  ventral  border  of  the  hippocampal  formation ;  but  I  have  thought 
it  simpler  (and  more  in  keeping  with  corresponding  conditions  of  the 
lateral  wall)  to  carry  it  across  the  base  of  Beccari's  median  triangle, 
in  a  fissure  which  is  present  and  which  delimits  the  cellular  formation 
characteristic  of  this  region,  just  as  the  arcuate  fissure  does  laterally. 
In  the  more  rostral  sections  it  will  be  seen  that  the  median  part  of  the 
rhinal  arcuate  fissure  is  much  shallower  than  the  very  sharp  one  lying 
immediately  below  it  within  the  tuberculum.  The  two  fissures  define 
a  rather  prominent  rolled  portion  of  the  tuberculum  which  is  probably 
to  be  identified  with  the  nucleus  of  the  medial  olfactory  tract  of  Livini 
(1908)  and  others.  Above  the  rhinal  arcuate  fissure  the  characteristic 
formation  of  the  tuberculum  falls  away  from  the  surface  and  runs  up 
along  the  median  border  of  the  nucleus  accumbens  (nuc.  ac.,  Fig.  30), 
almost  if  not  quite  to  the  ventricular  ependyma  beneath  the  more  ros- 
tral portion  of  the  body  of  the  anterior  commissure,  where  the  latter 
breaks  across  the  ventricle  to  reach  the,  septum.  It  thus  intervenes  be- 
tween the  sharply  defined  nucleus  accumbens  and  the  ventral  portion  of 
the  precommissural  body  or  septal  formation.  Its  own  medial  boundary 
is  also  sharply  defined  from  the  septal  formation.  The  tuberculum  ap- 
parently receives  secondary  olfactory  fibers  from  both  the  lateral  and 
medial  olfactory  tracts  (Fig.  28),  whose  fibers  may  be  seen  bending 
down  into  the  plexiform  layer  of  the  tuberculum  on  its  lateral  and 
medial  borders.  Beccari  (1910)  questions  this  (see  page  190  above), 
finding  evidence  of  other  sources  of  origin  (the  pyriform  lobe  in  par- 
ticular) for  the  fibers  of  the  external  plexiform  layer  of  the  tuberculum. 
He  thinks  that  the  olfactory  tract  fibers,  if  present,  exist  only  rostrally 
in  this  layer.  It  looks  otherwise  here,  but  this  is  not  really  decisive 
material. 

In  front  of  the  strio-tubercular  fusion  (see  Figs.  29-30),  which 
takes  place  behind  the  caudal  end  of  the  posterior  part  of  the  anterior 
olfactory  nucleus  (see  page  191)  essentially  in  the  fashion  so  clearly 
described  by  Livini  ( 1908)  in  Hypsiprymnus,  a  rather  wide  deep  plexi- 
form layer,  continuous  with  the  superficial  encircling  plexiform  of  the 
section,  intervenes  between  the  tuberculum  and  the  rest  of  the  cellular 
formation  of  the  section.  This  layer  is  crowded  with  a  wealth  of  fibers 
of  diverse  origin  and  destination,  whose  adequate  analysis  is  impossible 
in  this,  series.  They  include  fibers  from  the  lateral  olfactory  tract,  an- 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      197 

terior  olfactory  nucleus,  intermediate  olfactory  tract  (probably  both 
crossed  and  uncrossed),  from  the  tuberculum,  which  are  destined  for 
the  septal  nuclei,  hippocampus,  frontal  neopallial  pole.  Probably  all  of 
these  systems  are  to  some  extent  doubly  oriented.  And  some  of  them 
also  apparently  contribute  to  the  median  forbrain  bundle,  fasciculus 
medialis  telencephali  (f.med.t.,  Figs.  29-35),  which  begins  collecting 
in  the  base  of  the  septum  quite  far  forward ;  these  fibers  are  projection 
fibers  to  stem  centers  and  probably  from  them  also.  It  is  the  mul- 
titudinous ascending  precommissural  fibers  which  are  mainly  responsi- 
ble for  the  differentiation  of  the  rostral  portion  of  the  hippocampus  in 
lower  mammals. 

The  histological  development  of  the  tuberculum  is  in  Caenolestes 
spectacular  in  the  highest  degree.  The  external  cell  layer  of  medium 
darkstaining  pyramidal  or  polymorphic  cells  is  thrown  into  battlement- 
like  folds,  interrupted  irregularly  by  islands  of  Calleja  (i.  C.,  Figs.  29- 
32),  composed  of  extremely  small,  pale,  round  cells  densely  crowded, 
glomerulus-like  roundish  or  vermiculate  areas,  and  including  sometimes 
a  few  pale  giant  cells.  These  masses  vary  greatly  in  size  and  shape,  and 
they  are  so  sharply  delimited  and  so  different  from  their  surroundings 
as  to  suggest  pathological  growths.  They  probably  correspond  to 
Beccari's  (1910)  Type  2  islands,  while  deeper  ones  of  the  same  general 
character  belong  to  his  Type  3  islands.  The  Type  I  islands  consist  of 
thickenings  of  the  crenulations  of  the  external  cell  layer ;  these  are  some- 
times fringed  with  pale  granules  like  those  of  the  other  islands.  While 
these  types  are  sharply  differentiated,  there  are,  as  Becarri  found, 
intermediate  types.  Small  isolated  cell  masses  in  the  external  plexiform 
layer  of  the  more  caudal  sections  especially  are  all  traceable  into  the 
main  mass  of  the  tuberculum.  None  of  Beccari's  figures  show  so  great 
a  histological  complexity  as  Caenolestes.  The  conditions  in  the  Virginia 
opossum  are  much  less  complex  (Gray,  1924). 

What  is  the  function  of  these  highly  elaborated  and  integrated  organs 
icithin  an  organ?  They  suggest  some  sort  of  elaborate  rehandling  and 
sorting  of  incoming  stimuli — a  physiological  analysis  by  means  of  differ- 
ential thresholds,  effecting  a  secondary  specificity  from  mass  stimuli  dis- 
sociated, reenforced  and  more  or  less  independently  projected?  A 
searching  study  of  the  tuberculum  at  its  most  bizarre  stage  of  develop- 
ment in  the  lower  mammals  ought  surely  to  discover  valuable  clues  to 
modes  of  nervous  organization. 

The  deep  layers  of  the  tubercular  "cortex"  are  best  described  in  con- 
nection with  the  immense  fiber  and  giant  cell  stream  which,  like  a  great 
diagonally  slung  hammock,  extends  from  the  ventral  pyriform  cortex 


198  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

and  lateral  anterior  commissure  region  across  the  base  of  the  hemi- 
sphere to  the  ventro-medial  "regio  innominata",  the  vestibule  to  the 
thalamus  and  lower  stem  centers.  This  great  and  complex  system, 
composed  of  an  immense  number  and  variety  of  fibers,  both  pro- 
jection and  associational,  is  certainly  one  of  the  striking  features  of 
the  middle  subventricular  region  of  the  hemisphere.  It  is  generally  called 
the  basal  olfactory  bundle  (of  Wallenberg)  in  these  lower  mammalian 
brains.  It  forms  a  part  of  the  practically  unanalyzable  mass  of  fibers 
(in  this  series)  traversing  the  basal  region  of  the  hemisphere,  and  in- 
cluding such  systems  as  the  olfactory  projection  of  Cajal,  striatal 
systems  (ansa  lenticularis),  etc.,  the  whole  mass,  save  the  association 
fibers,  drifting  ventro-medially  to  join  the  medial  forebrain  bundle 
and  continue  spinalward  with  it.  Therefore,  following  some  writers, 
I  have  called  it  the  lateral  limb  of  the  medial  forebrain  bundle  (fasci- 
culus medialis  telencephali,  pars  lateralis,  f.med.t.1.,  Figs.  29-33).  It 
is  largely,  in  all  probability,  a  doubly  oriented  system.  Its  more  ros- 
tral portion  is  characterised  by  the  presence  of  a  great  multitude  of 
pale  giant  cells  strewn  among  the  fibers  (nucleus  of  the  basal  olfactory 
bundle,  Figs.  29-33).  A  similar  condition  occurs  in  the  more  median 
fiber  tangle,  whose  giant  cells  correspond  probably  to  the  "border 
nucleus"  of  Volsch  (1906).  The  ansa  lenticularis  component  contains 
few  cells.  But  for  many  of  its  fibers  the  cells  of  origin  are  the  giant 
cells  characteristic  of  the  globus  pallidus  (gl.p.,  Figs.  12,  15;  glob,  p., 
Fig.  34),  and  these  are  just  like  those  strewn  so  thickly  among  the  fibers 
of  the  more  rostral  stream  of  the  medial  forebrain  bundle,  especially 
in  its  lateral  limb  and  in  the  more  medial  portion,  where  they  seem  to 
correspond  to  the  "border  nucleus"  of  Volsch  (1906).  The  globus 
pallidus  cells  are  certainly  motor  projection  cells,  and  very  likely  the 
other  giant  cells  mentioned  also  send  long  axons  to  stem  centers.  The 
fact  that  the  red  nucleus  is  also  mainly  composed  of  pale  giant  cells  of 
the  same  type  further  tends  to  support  this  supposition. 

With  the  reduction  of  the  lateral  and  basal  olfactory  centers  in 
higher  mammalian  brains,  the  more  posterior  ansa  lenticularis  com- 
plex becomes  so  preponderant  as  to  throw  the  more  anterior  olfactory 
complex  into  the  shade,  and  so  the  name  "ansa  lenticularis"  comes  to 
be  applied  to  the  whole  stream.  But  in  lower  mammalian  brains  it  is 
the  more  anterior  component,  mediating  correlated  olfactory  stimuli, 
which  seems  more  conspicuous.  Many  longitudinal  fibers  and  fiber 
bundles  (association  tracts  between  rostral  and  caudal  regions  of  the 
base  of  the  hemisphere)  further  complicate  the  situation. 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      199 

Septal  region. — The  precommissural  area  of  the  median  surface  of 
the  hemisphere  is  the  external  surface  of  the  thick  paraterminal  body 
of  Elliot  Smith  or  septum  of  ordinary  terminology.  This  region  con- 
tains the  ascending  olfactory  systems  already  enumerated  (olfacto- 
septal,  olfacto-cortical)  and  the  descending  precommissural  fornix, 
septo-amygadaloid  (Johnston's  stria  terminalis  bundle  4),  septo-habe- 
nular  (of  stria  medullaris  system)  and  median  forebrain  bundles.  The 
bundle  labelled  olfacto-f rental  (tr.ol.fr.,  Fig.  27),  extending  from  the 
frontal  pole  of  the  neopallium  into  the  septum,  where  it  mingles  with 
the  septal  fibers  which  sweep  laterally  beneath  the  ventricle  or  below 
the  lateral  mass  of  the  anterior  commissure,  cannot  itself  be  disen- 
tangled and  followed  with  precision  in  these  sections.  Arising  in  the 
frontal  pole  of  the  neopallium  just  in  front  of  the  laterally  directed 
corona  radiata  fibers  destined  for  the  internal  and  external  capsules, 
these  fibers  inevitably  suggest  the  possibility  of  part  of  the  coronal 
fibers  being  diverted,  or  rather  persisting,  medially  into  the  septum  as 
projection  fibers  to  stem  centers  by  way  of  the  median  forebrain  bundle, 
as  a  vestige  of  what  Edinger  has  called  the  septomesencephalic  tract 
of  submammalia.  In  this  connection  Pedro  Cajal's  findings  (1917, 
1919)  in  Varanus  and  Lacerta,  of  a  septal  passage  for  descending  fibers 
from  the  entire  cortex,  including  the  depressed  portion  between  the 
medio-dorsal  hippocampal  cortex  and  the  lateral  pyriform  cortex,  which 
he  considers  the  "general  cortex",  seem  highly  significant.  One  is  led 
to  recall  also  that  the  only  cortical  projection  tract  in  birds  is  a  septal 
one,  and  that  it  does  not  proceed  from  what  seems  to  be  olfactory 
cortex.  It  is  not  inconceivable  that  in  the  neopallial  frontal  pole  of 
the  lowly  mammalian  type  of  brain,  at  a  level  where  the  hippocampal 
formation  is  still  rather  insignificant,  the  more  medial  neopallial  fibers 
might  have  preserved  the  shorter  septal  path. 

Two  well  developed  nuclei  lie  in  the  septum — the  lateral  and  medial 
parolfactory  nuclei  (Herrick).  The  lateral  parolfactory  nucleus 
(nuc.pol.L,  Figs.  28-30)  occupies  the  septal  wall  lateral  to  the  pre- 
commissural fornix  fibers.  It  corresponds  to  Johnston's  (1913)  pri- 
mordium  hippocampi  and  not  to  his  lateral  parolfactory  nucleus,  which 
is  here  the  nucleus  accumbens  (nuc.ac.},  the  medial  portion  of  the 
head  of  the  caudate  nucleus.  The  medial  parolfactory  nucleus  (nuc. 
pol.m.,  Figs.  30-31),  located  medially  and  ventrally,  corresponds  to 
Johnston's  riucleus  of  the  same  name,  and,  as  he  finds  in  the  Virginia 
opossum  and  in  other  forms  (Johnston,  1923),  it  passes  back  insen- 
sibly into  the  nucleus  of  the  diagonal  band  of  Broca  (nuc.d.b.,  Figs. 


2OO  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

32-33).     Apparently  it  grades  also  into  the  preoptic  nucleus  of  the 
telencephalon  medium  (nuc,  prop.,  Fig.  34). 

The  dorsal  (d.)  and  anterior  (c.a.)  commissures  have  already  been 
briefly  described  in  connection  with  the  median  section  of  the  brain. 
The  dorsal  commissure  will  be  considered  also  below  in  connection 
with  the  hippocampus.  The  composition  of  the  anterior  commissure 
offers  in  Caenolestes  nothing  of  unusual  interest.  But  in  view  of  the 
disagreement  among  zoologists  with  regard  to  the  assignment  of  Caen- 
olestes  to  one  or  the  other  of  the  two  marsupial  subgroups — Diproto- 
dontia  and  Polyprotodontia — with  its  bearing  on  marsupial  distribu- 
tional problems,  the  arrangement  of  the  fibers  of  the  anterior  commis- 
sure was,  since  it  seemed  to  be  the  only  remaining  anatomical  evidence 
to  be  expected  on  the  question,  a  matter  of  great  interest  (Obenchain, 
19233,  I923b).  After  an  examination  of  the  brains  of  every  mar- 
supial genus  except  Caenolestes  (which  was  not  available)  Elliot  Smith 
(i9O2a  and  b)  found  that  all  diprotodont  brains  exhibit  one  feature 
never  found  in  any  polyprotodont  brain,  or  indeed  in  any  other  ver- 
tebrate brain.  This  exclusive  diprotodont  character  he  named  the 
aberrant  bundle  (fasciculus  aberrant)  of  the  anterior  commissure, 
considering  it  a  true  diagnostic  character  of  diprotodont  brains.  It  is 
merely  the  dorsal  portion  of  the  anterior  commissure  which  in  dipro- 
todonts  splits  off  from  the  rest  of  the  commissure  to  pass  upward  by 
way  of  the  internal  instead  of  the  external  capsule,  the  common  route 
in  all  other  brains.  The  aberrant  bundle  is  absent  in  Caenolestes,  a 
fact  which,  if  this  feature  be  decisive,  would  ally  it  with  the  polypro- 
todonts.  The  brains  of  the  fossil  caenolestids,  however,  can  never  be 
known,  but  the  presumption  of  the  absence  of  the  aberrant  bundle  in 
them  also  would  perhaps  be  justifiable.  The  effect  of  this  would  occa- 
sion no  further  disturbance  of  Dr.  Osgood's  marsupial  "family  tree" 
than  the  lengthening  of  the  polyprotodont  bracket  to  include  Caeno- 
lestes, leaving  it  still  in  place  between  the  generalised  polyprotodont 
Perameles  and  the  diprotodonts  of  Australia;  or  at  most,  in  view  of 
the  intermediate  position  of  the  exposed  gyrus  dentatus  of  Caenolestes, 
the  latter  might  be  shifted  to  a  position  between  Perameles  and 
Notoryctes. 

Pyriform  Lobe*. — The  pryiform  lobe  consists  of  two  distinct  parts : 
(i)  the  pyriform  cortex,  mostly  confined  to  lateral  wall  of  the  hemi- 

*The  limits  here  assigned  to  the  pyriform  lobe  do  not  exactly  coincide  with 
those  assigned  either  by  Cajal  or  Johnston,  although  the  actual  descriptions 
of  the  areas  involved  vary  little  or  not  at  all.  Cajal  (1911)  restricts  the 
pyriform  lobe  to  include,  besides  the  amygdala,  only  the  median  pyriform  cortex 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.     201 

sphere,  between  the  rhinal  fissure  above  and  the  rhinal  arcuate  and 
amygdaloid  fissures  below;  and  (2)  the  amygdaloid  complex,  mostly 
in  the  ventral  wall  between  the  amygdaloid  and  choroid  fissures. 

The  amygdaloid  complex,  thanks  to  Johnston's  illuminating  com- 
parative analysis  (1923),  which  builds  upon  and  completes  Volsch's 
intensive  study  (1906),  is  no  longer  the  mysterious  territory  it  has 
been.  Since  it  is,  strictly  speaking,  a  subcortical  center,  it  will  be 
described  first  here,  leaving  the  pyriform  cortex  to  precede  the  hippo- 
campus. Owing  to  its  enormous  extent,  the  amygdaloid  complex  is 
perhaps  the  most  spectacular  part  of  the  brain — unless  it  share  this 
distinction  with  the  tuberculum  olfactorium.  The  superficial  extent 
of  the  amygdaloid  complex  has  already  been  described.  Internally  it 
exceeds  this  both  in  length  and  width,  overlapping  the  posterior  por- 
tion of  the  tuberculum  anteriorly  and  the  pyriform  cortex  laterally, 
and,  as  Johnston  (1923)  points  out  in  the  Virginia  opossum,  stretch- 
ing rostrally  far  towards  the  anterior  commissure.  Behind  the  caudate 
nucleus  and  the  putamen  (put.)  it  floors  the  ventricle,  while  the  cellu- 
lar bed  of  its  great  fiber  system,  the  stria  terminalis  (st.t.,  Figs.  32  ff.) 
forms  the  median  strip  of  the  floor  behind  the  anterior  commissure. 
I  am  not,  however,  able  to  follow  the  stria  bed  in  these  sections  into 
the  anterior  olfactory  nucleus,  as  Johnston  does  in  the  opossum.  It 
is  clear  anteriorly  here  only  as  it  lies  upon  the  anterior  commissure 
and  more  caudally  upon  the  internal  capsule  or  cerebral  peduncle. 

The  nuclei  of  the  amygdaloid  complex  in  Caenolestes  comprise  the 
six  described  by  Johnston  (1923),  and  include  also  the  extra  seventh 
one  he  found  in  the  Virginia  opossum :  the  nucleus  of  the  lateral  olfac- 
tory tract,  the  central,  medial,  lateral,  basal,  accessory  basal,  and  corti- 
cal amygdaloid  nuclei. 

(his  anterior  pyriform  cortex),  receiving  terminals  of  the  lateral  olfactory 
tract,  and  the  posterior  pyriform  cortex,  receiving  no  direct  olfactory  fibers 
(his  superior  temporal  cortex).  This  makes  it  coincide  approximately  with 
the  gyrus  hippocampi  of  primates.  Johnston  (1915)  defines  the  pyriform  lobe 
as  the  gray  matter  underlying  the  lateral  olfactory  tract,  which  would  include 
the  entire  anterior  olfactory  nucleus,  but  not  the  posterior  pyriform  cortex. 
By  this  definition  the  pyriform  lobe  and  hippocampus  are  directly  continuous 
anteriorly,  but  not  in  contact  posteriorly,  since  the  posterior  pyriform  cortex 
would  be  included  in  the  neopallium.  Since  this  area  is  the  field  of  origin  of  the 
temporo-ammonic  tract  (Cajal),  the  main  afferent  tract  to  the  hippocampus  in 
mammals,  it  seems  more  logical  to  include  it  within  the  pyriform  lobe.  Its 
exclusion  by  Johnston  would  explain  the  complete  caudal  separation  of  the 
pyriform  lobe  and  hippocampus  in  the  turtle,  Cistudo  Carolina  (1915).  The 
anterior  pyriform  cortex  here  coincides  with  Cajal's  "frontal  lobe",  which  he 
excludes  from  the  pyriform  lobe,  on  the  basis  that  it,  like  the  peduncular 
gray  (anterior  olfactory  nucleus)  receives  mainly  collaterals  of  the  lateral  ol- 
factory tract. 


202  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

The  well  delimited  nucleus  of  the  lateral  olfactory  tract  (nuc.tr. ol.L, 
Figs.  12,  15,  36-38)  is  a  duplex  structure,  consisting  of  a  large-celled 
dorsal  part  and  a  small-celled  ventral  part,  separated  by  a  narrow 
plexiform  layer.  It  is  tilted  up  rostrally,  and  shifted  forward  for  about 
half  its  length  above  the  caudal  portion  of  the  tuberculum,  and  there- 
fore only  its  posterior  half  lies  beneath  the  medio-ventral  tubercle  which 
in  Caenolestes  is  apparently  not  so  pronounced  as  in  Orolestes,  of 
which  no  sections  are  now  available  for  comparison.  The  nucleus  of 
the  diagonal  band  (nuc.d.b.,  Figs.  32,  33)  curves  around  from  the 
median  wall  here,  and  the  diagonal  band  fibers  pass  laterally  in  the 
diffuse  region  above  the  posterior  end  of  the  nucleus  of  the  lateral 
tract  towards  the  pyriform  cortex,  probably  also  effecting  connections 
with  the  intermediate  region.  The  ventral  small-celled  portion  of  the 
nucleus  is  so  sharply  delimited  as  to  appear  almost  encapsulated,  an 
appearance  heightened  at  its  caudal  pole,  which  extends  beyond  the 
dorsal  part  as  a  sort  of  island  of  cells  in  the  plexiform  layer  of  the 
hemisphere.  The  corresponding  n'ucleus  of  the  Virginia  opossum  ex- 
hibits neifher  the  forward  shifting  above  the  tuberculum  nor  the 
histological  differentiation  seen  in  Caenolestes  (Gray,  1924).  The 
nucleus  of  the  lateral  olfactory  tract  receives  secondary  olfactory  fibers 
from  the  pars  ventralis  of  the  lateral  olfactory  tract  (unlabelled,  Figs. 
36-38),  and  dorsally  a  great  fan  of  fibers  from  the  stria  terminalis 
(st.t.i,  Figs.  33-36),  its  most  anterior  contingent.  Johnston  (1923) 
identifies  this  with  the  commissural  bundle  of  the  stria,  but  these  sec- 
tions do  not  actually  permit  this. 

The  central  nucleus  of  the  amygdala  (c.,  Figs.  12,  15;  nuc.amg.c., 
Figs.  35-37)  lies  above  the  nucleus  of  the  lateral  olfactory  tract.  It 
is  confluent  with  the  strongly  developed  "intercalary  plate"  (int. plate, 
Fig.  36)  of  Johnston  (1923),  which  is  the  most  hypertrophied  part 
of  the  stria  bed.  The  central  n'ucleus  advances  farther  forward  than 
any  other  amygdaloid  nucleus,  but  apparently  not  so  far  as  Johnston 
found  in  the  Virginia  opossum,  up  to  the  region  of  the  anterior  com- 
missure. Its  limits  are  not  well  defined  nor  its  cellular  structure  strik- 
ing. 

The  medial  amygdaloid  nucleus  («*.,  Figs.  12,  15;  nuc.amg.m.,  Figs. 
36,  37)  occupies  the  ventro-medial  angle  of  the  hemisphere,  and  also 
lacks  well  defined  limits  or  strikingly  marked  cell  structure.  It  is 
impossible  to  fix  its  ca'udal  limit  here.  It  also  is  continuous  with  the 
intercalary  plate  and  with  the  central  nucleus,  as  Johnston  found. 

The  lateral  amygdaloid  nucleus  (I.,  Figs.  12,  15;  nuc.amg.1.,  Figs. 
35-39)  is  the  "poststriatum"  of  earlier  writers.  It  is  a  large  and  ex- 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.     203 

tremely  well  defined  antero-posteriorly  elongated  nucleus,  oval  in  sec- 
tion and  characterized  by  good-sized  pyramidal  or  polymorphic  cells. 
It  lies  in  the  concavity  of  the  external  capsule  (cap.e),  between  it  and 
the  ventral  part  of  the  putamen  (put.),  and  behind  the  latter  it  rises  to 
form  the  more  lateral  part  of  the  ventricular  floor.  It  clearly  receives 
external  capsule  fibers,  more  probably  originating  in  the  pyriform  cor- 
tex, and  it  seems  also  to  receive  a  large  number  of  stria  terminalis  fibers, 
coursing  horizontally  just  above  the  basal  amygdaloid  nucleus  (st.t.  3, 
Fig.  38).  This  last  is  contrary  to  Johnston's  observations. 

The  large-celled  well  defined  basal  nucleus  of  the  amygdala  (&., 
Figs.  12,  15;  nuc.amg.b.,  Figs.  37-42)  arises  medial  to  the  more  caudal 
portion  of  the  lateral  nucleus,  near  its  ventral  border,  and  increases  to 
large  proportions  as  the  lateral  nucleus  diminishes.  Behind  the  flat- 
tened tail  of  the  caudate  nucleus  it  occupies  a  large  part  of  the  floor 
of  the  ventricle,  extending  far  medialward  and  lying  caudally  directly 
beneath  the  ependyma.  Both  it  and  the  lateral  nucleus  are  in  cellu- 
lar continuity  with  the  deep  cells  of  the  ventral  border  of  the  pyri- 
form cortex  at  the  level  of  the  amygdaloid  fissure.  Johnston  (1923) 
considers  them  to  have  been  derived  from  the  pyriform  cortex  by  a 
process  of  infolding  along  this  line,  and  the  situation  in  Caenolestes 
seems  to  support  this  view. 

The  accessory  basal  nucleus  of  the  amygdala  (b.ac.,  Fig.  15;  nuc. 
amg.b.ac.,  Figs.  37-38)  is  a  less  clearly  defined  nucleus  of  medium 
dark  cells  lying  among  the  ventral  fibers  of  the  external  capsule  as 
they  fan  out  in  the  postero-ventral  amygdaloid  region,  below  the 
caudal  ends  of  the  lateral  and  basal  nuclei  and  obscurely  confluent  with 
them.  It  is  not  only  present  in  Caenolestes,  but  it  would  be  only  too 
easy  to  subdivide  the  heterogeneous  caudal  amygdaloid  region  into 
further  nuclei. 

The  cortical  amygdaloid  nucleus  (amg.,  Fig.  12;  nuc.amg.cort., 
Figs.  38-43)  forms  almost  the  entire  superficial  portion  of  the  amygda- 
loid complex.  Laterally  it  approaches  or  adjoins  the  ventral  edge  of 
the  pyriform  cortex  and  medially  the  ventral  edge  of  the  hippocampus 
behind  the  choroid  fissure  (fs.ch.)  along  an  internally  well  marked 
and  an  externally  partially  obvious  medial  continuation  of  the  amygda- 
loid fissure  (fs.amg.m).  Its  histological  structure  assumes  in  places 
a  structure  similar  to  that  of  the  pyriform  cortex.  Its  deeper  portion 
is  very  heterogeneous.  Some  confusion,  however,  results  from  the 
tangential  nature  of  the  posterior  sections  of  the  hemisphere.  The 
cortical  amygdaloid  nucleus  receives  secondary  fibers  by  way  of  the 
pars  ventralis  of  the  lateral  olfactory  tract.  The  hippocampal-amygda- 


2O4  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

loid  junction  (subic.  (amg.),  Figs.  40-42)  along  the  median  extension 
of  the  amygdaloid  fissure  is  also  evidence  of  fiber  connections,  and 
Johnston  (1923)  has  demonstrated  them  in  the  Virginia  opossum. 

The  stria  terminalis  (st.t.,  Figs.  32-38),  the  great  fiber  system  re- 
lated to  the  amygdaloid  complex,  has  also  been  carefully  analyzed  by 
Johnston  (1923).  Practically  its  entire  course  is  quite  clear  in  these 
sections,  and  rostrally  and  caudally  its  five  component  bundles  as  iden- 
tified by  Johnston,  can  easily  be  recognized,  but  in  the  compact  medial 
portion  of  the  tract,  where  it  lies  in  the  stria  bed  between  the  cerebral 
peduncle  and  the  ependyma  of  the  lateral  ventricle  they  cannot  be  in- 
dividually identified.  Johnston's  numbers  have  been  affixed  to  these 
bundles  as  traced  by  him  in  the  Virginia  opossum.  Rostrally  they  are 
certainly  correctly  applied,  but  caudally  they  are  applied  without  direct 
evidence  from  the  brain  of  Caenolestes  of  continuity  with  the  respective 
rostral  bundles. 

The  lateral  part  of  the  anterior  olfactory  nucleus  passes  without 
break  directly  over  into  the  least  differentiated  anterior  pyriform  cor- 
tex behind  it.  The  cortex  of  the  pyriform  lobe  covers  most  of  the 
lateral  surface  of  the  pyriform  lobe.  Antero-laterally  it  may  be  divided 
into  three  regions:  the  anterior  pyriform  cortex  (cx.pir.a.,  Figs.  13, 
15,  26-31;  area  piriformis  anterior,  Gray,  1924;  frontal  lobe  of  Cajal, 
1911),  which  passes  more  caudally  by  very  gradual  transition  into  still 
more  differentiated  medial  pyriform  cortex  (cx.pir.m.,  Figs.  13,  15, 
32-34;  area  piriformis  medialis,  Gray;  anterior  pyriform  cortex  of 
Cajal;  this  in  turn  merges  more  abruptly  into  the  most  specialized 
posterior  pyriform  cortex  (cx.pir.p.,  Figs.  12,  13,  15,  42-44;  area 
piriformis  posterior,  Gray;  superior  temporal  or  angular  nucleus  or 
center  of  Cajal.  Since  the  lateral  part  of  the  anterior  olfactory 
nucleus  merges  insensibly  with  the  anterior  pyriform  cortex,  and  the 
latter  merges  insensibly  into  the  medial  pyriform  cortex,  no  definite 
boundaries  can  be  made  out  between  them.  The  second  cortical  layer 
(not  counting  the  external  fibrillar  layer)  becomes  progressively  more 
and  more  condensed,  and  any  lines  of  demarcation  are,  in  these  sections, 
merely  arbitrary.  But  the  posterior  pyriform  cortex  (cx.pir.p.,  Figs. 
42-44),  which  is  on  a  different  plane  functionally  from  the  remainder 
of  the  pyriform  cortex  because  (by  Cajal's  definition)  it  receives  no 
secondary  olfactory  fibers,  also  differs  histologically.  Its  histological 
development  is  connected  not  only  with  the  absence  of  secondary  olfac- 
tory fibers,  but  perhaps  even  more  with  the  increase  of  non-olfactory 
fibers,  and  its  consequent  elevation  into  an  associational  area  second 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.     205 

in  rank  only  to  the  neopallium.  It  displays  a  very  striking  anatomical 
character.  This  is  mainly  due  to  the  development  within  it  of  a  wide 
and  very  dense  plexus  in  the  third  layer  of  the  cortex.  In  Caenolestes 
this  plexus,  which  according  to  Cajal  is  in  the  mouse  of  extraordi- 
nary density,  almost  fills  the  pyriform  wall  dorso-caudally.  Its  broad, 
rounded  head  rises  slightly  above  the  level  of  the  rhinal  fissure.  It 
contains  many  pale  giant  cells,  found  nowhere  else  in  the  pyriform 
cortex  of  Caenolestes.  The  posterior  pyriform  region  is  the  field  of 
origin  of  the  great  temporo-ammonic  or  angular  bundle  of  Cajal 
(1911,  spheno-ammonic,  1906),  which  delivers  a  huge  stream  of  highly 
correlated  olfacto-somatic  impulses  to  both  the  ammon's  horn  and  the 
gyrus  dentatus,  for  almost  their  entire  length — certainly  reaching  as 
far  forward  in  Caenolestes  as  the  anterior  level  of  the  commissural 
region,  where  its  structural  influence  is  suddenly  and  strikingly  felt. 
Since  it  mingles  with  other  pyriform  and  with  neopalial  association 
fibers  to  form  the  cingulum  limitans  (ci.lim.,  Figs.  3042)  at  the  inner 
dorsal  angle  of  the  lateral  ventricle,  and  the  cingulum  ammonis  (ci.am., 
Figs.  30-42)  in  the  outer  plexiform  layers  of  the  ammon's  horn  and 
gyrus  dentatus,  it  is  not  separately  named  in  the  sections  given  here. 
In  the  posterior  region  of  the  hemisphere  this  avalanche  of  fibers  may 
be  seen  pouring  above  and  behind  the  ventricle  into  the  presubicular 
and  subicular  regions  and  through  them,  either  directly  or  indirectly, 
as  perforating  fibers  to  the  ammon's  horn  and  gyrus  dentatus. 

Anteriorly  the  lateral  olfactory  nucleus  certainly  sends  tertiary  ol- 
factory fibers  above  the  olfactory  ventricle  into  the  anterior  hippo- 
campal  formation  (Figs.  26-27).  The  sections  suggest  that  the  dorsal 
path  between  the  pyriform  and  hippocampal  cortices  might  perhaps 
be  patent  for  practically  the  entire  length  of  the  neopallium,  in  the 
deep  layer  of  the  corona  radiata.  Owing  to  the  height  of  the  rhinal 
fissure  the  distance  is  nowhere  very  great,  and  this  would  certainly  be 
the  shortest  path  for  the  more  dorsal  pyriform  cortex.  The  anterior 
pyriform  cortex  must  also  contribute  very  largely  to  the  subventricular 
systems,  both  to  the  olfacto-septal,  olfacto-hippocampal  and  olfacto- 
f rontal  systems  anteriorly,  and  to  the  more  posterior  and  extensive  com- 
plex included  under  the  head  of  the  lateral  limb  of  the  medial  forebrain 
bundle,  as  explained  above  (see  page  198).  The  great  efferent  pyri- 
form path  to  stem  centers  is  of  course  the  olfactory  projection  path 
of  Cajal,  probably,  like  so  many  of  these  fiber  systems,  a  doubly 
oriented  one.  The  stria  terminalis  component  of  this  system  (John- 
ston's bundle  2,  1923)  arises  from  the  amygdaloid  complex  (so  far 
as  it  is  a  descending  bundle).  The  fiber  connections  between  the 


206  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

pyriform  cortex  and  the  amygdaloid  complex  have  already  been  men- 
tioned (page  203). 

In  addition  to  the  antero-posterior  divisions  of  the  pyriform  cortex, 
Gray  (1924)  has  described  several  narrow  longitudinal  bands  along  the 
boundary  fissures  in  the  Virginia  oppossum.  Three  such  areas  accom- 
pany the  rhinal  fissure:  the  area  perirhinalis,  the  area  piriformis  dor- 
salis,  and  the  area  piriformis  fissuralis,  in  order  from  above  down- 
wards. The  extreme  shallowness  of  the  rhinal  fissure  -in  Caenolestes 
is  not  conducive  to  great  development  of  these  areas.  The  area  peri- 
rhinalis, the  transition  area  between  neopallial  and  pyriform  cortex, 
is  probably  individualized,  but  the  other  two  are  feebly  developed  and 
not  distinct  from  one  another.  Ventrally  also  the  area  piriformis 
ventralis  and  the  area  subpiriformis  of  Gray  tend  to  fuse  into  one 
band  quite  well  marked,  the  ventral  pyriform  cortex  (cx.pir.v.,  Figs. 
27-34),  especially  anteriorly  where  it  describes  a  deep  reentering  angle 
subjacent  to  the  massive  lateral  olfactory  tract.  This  band  is  here 
regarded  as  following  caudally  upon  the  latero-ventral  part  of  the 
anterior  olfactory  nucleus  (nuc.ol.ant.l.v.,  Fig.  26).  Behind  the  tuber- 
culum,  and  even  more  rostrally,  where  the  diffuse  anterior  perforated 
space  (l.perf.a.,  Figs.  35-38)  of  Johnston  (1923)  intervenes  between 
it  and  the  nucleus  of  the  lateral  olfactory  tract,  the  pyriform  cortex 
exhibits  a  very  sharp  ventral  margin  back  to  the  point  where  it  gives 
evidence  of  infolding  at  the  level  of  the  lateral  and  basal  amygdaloid 
nuclei.  Still  further  back  it  becomes  more  or  less  continuous  with  the 
outer  layer  of  the  cortical  amygdaloid  nucleus,  and  histological  differen- 
tiation tends  to  fade  out.  The  tangential  nature  of  the  sections  in  this 
caudal  region  is  somewhat  confusing,  and  control  by  sections  in  other 
planes  is  unfortunately  lacking. 

HIPPOCAMPAL  FORMATION 

The  hippocampal  formation  of  Caenolestes  (Figs.  6,  12,  17,  26-43) 
begins  with  a  small  patch  of  cells  condensed  upon  the  outer  aspect  of 
the  rostral  border  of  the  anterior  olfactory  nucleus,  at  its  dorso-medial 
angle,  that  is  to  say,  immediately  caudal  to  the  b'ulbar  formation  (hip. a., 
Figs.  12,  17;  cx.hip.a.,  Figs.  26-29).  It  thus  takes  part  in  the  forma- 
tion of  the  rhinencephalic  ring  surrounding  the  olfactory  ventricle,  by 
means  of  which  the  two  olfactory  cortices  are  indirectly  continuous 
with  one  another  through  the  anterior  olfactory  nucleus.  It  is  to  be 
noted  that  while  the  hippocampal  formation  begins  at  the  very  rostral 
margin  of  this  ring,  in  its  intrabulbar  portion,  just  caudal  to  the  olfac- 
tory formation  itself,  a  similar  lateral  (pyriform)  condensation  falls 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.     207 

short  of  this,  being  separated  from  the  bulbar  formation  by  uncorti- 
cated  anterior  olfactory  nucleus.  The  explanation  of  this  precocity 
of  hippocampal  differentiation  might  conceivably  be  tied  up  with  the 
prevailing  latero-ventral  drift  of  the  secondary  olfactory  fibers  toward 
the  lower  olfactory  correlation  centers  of  the  lateral  wall  and  base  of 
the  hemisphere — pyriform  lobe  and  tuberculum  olfactorium.  This 
trend  is  clearly  brought  out  in  the  bulb,  where  there  is  never  any 
considerable  accumulation  of  secondary  fibers  in  the  dorso-medial  re- 
gion, those  arising  there  apparently  for  the  most  part  making  as  rap- 
idly as  possible  for  the  laterally  and  ventrally  situated  lateral,  inter- 
mediate and  medial  olfactory  tracts.  The  only  exception  to  this  is 
the  presence  of  the  small  more  median  portion  of  the  pars  dorsalis  of 
the  lateral  olfactory  tract  (tr.oLd.in.,  Fig.  26),  which  may,  and  prob- 
ably does  (as  silver  preparations  indicate  in  the  Virginia  opossum, 
Herrick,  19243),  give  off  some  collaterals  to  the  hippocampal  forma- 
tion, which  is,  however,  already  strongly  under  the  influence  of  ter- 
tiary olfactory  fibers.  It  seems  but  natural  that  such  a  region,  receiv- 
ing only  a  minimum  of  direct  olfactory  fibers  and  beginning  almost, 
if  not  quite,  at  once  to  receive  tertiary  olfactory  fibers  from  the  more 
lateral  portions  of  the  anterior  olfactory  nucleus  (these  can  be  seen 
crossing  above  the  olfactory  ventricle)  and  presently  still  more  highly 
correlated  olfactory  stimuli  from  the  tuberculum  by  way  of  the  sep- 
tum, should  undergo  accelerated  differentiation  in  comparison  with  a 
region  preponderantly  invaded  by  secondary  olfactory  fibers  (see  page 
219). 

Dorsally  the  continuity  between  the  hippocampal  formation  and 
the  pars  dorsalis  of  the  anterior  olfactory  nucleus  becomes  transformed 
at  the  junction  between  the  neopallium  and  the  rhinencephalon  into  a 
continuity  between  the  hippocampus  and  the  neopallium  on  the  one 
hand,  and  between  the  pyriform  cortex  and  the  neopallium  on  the 
other  (Figs.  26-27).  Ventrally  the  hippocampal  formation,  at  first 
continuous  with  the  pars  posterior  of  the  anterior  olfactory  nucleus 
(nuc.ol.ant.p.,  Figs.  27-28),  breaks  away  from  the  latter  and  becomes 
separated  from  it  by  an  increasing  interval  of  diffusely  scattered  small 
cells  through  which  some  fine  pale  fibers  pass  from  the  deeper  septal 
region  to  the  plexiform  layer  (Fig.  29).  This  area  is  replaced  more 
caudally  by  the  medial  parolfactory  nucleus  (nuc.pol.m.,  Figs.  30-31). 

The  ventral  end  of  the  precommissural  hippocampus  (Fig.  29) 
thins  to  a  slender  needle  point  which,  as  it  recedes  upward  in  the  septal 
wall,  approaches  the  pial  surface.  This  slender  column  of  cells  widens 


208  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

above  very  quickly  as  it  merges  with  the  neopallial  gray.  It  soon 
begins  to  show  a  slight  concavity  toward  the  pial  surface  (fs.hip.,  Fig. 
29),  which  more  caudally  comes  to  expression  as  the  external  hippo- 
campal  fissure  (fs.hip.,  Fig.  30).  The  cells  diminish  in  size  and  in- 
crease in  density  from  above  downward,  to  the  sharply  pointed  ventral 
end  of  the  hippocampal  formation,  which  curves  upwards  and  back- 
wards in  line  with  the  ventral  (medial)  edge  of  the  definitive  gyrus 
dentatus  at  the  fimbrio-dentate  fissure  near  its  rostral  end  at  the  antero- 
dorsal  angle  of  the  hippocampal  commissure  (fs.fim.d.,  c.d.,  cx.dent., 
Figs.  30-31).  As  the  hemisphere  increases  in  height  the  hippocampal 
formation  lengthens,  the  upper  thicker  and  larger-celled  portion  more 
rapidly  than  the  lower,  so  that  it  comes  to  exceed  the  lower  smaller- 
celled  portion  in  length.  It  becomes  at  the  same  time  thinner  and 
denser.  It  is  interesting  to  note  that  rostrally  the  hippocampal  for- 
mation differentiates  more  rapidly  at  its  peripheral  (dentate)  border, 
and  that  differentiation  travels  upward,  the  orderly  condensed  files 
of  definitive  ammon's  pyramids  appearing  tardily.  Caudally  also  we 
find  the  cortex  dentatus  apparently  leading  the  way  in  the  formation 
of  the  recurved  temporal  pole  of  the  hippocampus  (see  page  215). 

When  I  was  making  the  first  Edinger  drawings  I  noted  a  curious 
breaking  up  or  disorganization  in  the  precomrnissural  hippocampal 
cell  plate,  due  to  the  loosening  up  and  paler  staining  of  some  of 
the  cells  of  the  upper  region  (*,  Figs.  12,  i/a).  This  rift  or  line 
of  fracture  I  interpreted  as  the  locus  of  the  interpositio  medialis, 
the  break  between  the  definitive  gyrus  dentatus  and  the  ammon's  horn. 
But  to  my  great  surprise  the  interpositio  medialis  formed  rather  sud- 
denly at  a  distinctly  lower  (more  ventral)  level  at  the  anterior  border 
of  the  dorsal  commissure  (ip.m.,  Fig.  31).  A  provisional  explanation 
of  this  puzzling  rift  in  the  cell  plate  suggested  itself  later,  however, 
in  connection  with  Cajal's  statement  (1911,  II.,  p.  754)  that  the  gyrus 
dentatus  and  the  more  ventral  half  of  the  ammon's  horn  (his  extra- 
ventricular  ammon's  horn  or  region  of  grand  pyramids)  seem  to  form 
an  indissoluble  anatomical  and  functional  unit,  owing  to  the  exclusive 
distribution  of  efferent  gyrus  dentatus  axons  (the  "mossy  fibers")  to 
the  extraventricular  ammon's  horn.  The  line  of  fracture  in  question 
might  presumably  be  interpreted,  then,  as  the  upper  limit  of  the  dis- 
tribution of  the  "mossy  fibers",  the  boundary  between  intra-  and  ex- 
traventricular ammon's  horn.  (For  a  provisional  explanation  of  its 
absence  in  the  supra-  and  postcommissural  ammon's  horn,  see  page 

209  below).     The  material  available  here  does  not  furnish  proof  of 
this  hypothesis,  which  is  offered  merely  as  a  suggestion. 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.     209 

At  the  level  of  the  anterior  face  of  the  dorsal  commissure  the  pre- 
commissural  hippocampus,  which,  as  Elliot  Smith  (i8o,6c)  long  ago 
pointed  out,  exhibits  in  marsupials  all  stages  of  development  to  be  seen 
in  the  reptilian  hippocampus,  suddenly  assumes  the  definitive  mam- 
malian appearance  of  double  interlocked  arcs  by  the  abrupt  formation 
of  the  interpositio  medialis  separating  the  ammon's  horn  and  the  gyrus 
dentatus,  and  the  synchronous  humping  up  of  the  latter  into  a  horse- 
shoelike  form  into  whose  ventral  concavity  the  terminal  lamina  (Levi, 
1904;  nucleus  fasciae  dentatae,  Elliot  Smith,  i896c)  of  the  ammon's 
horn  is  displaced.  The  explanation  of  these  sudden  changes  is  proba- 
bly three-fold:  external  pressure,  internal  pressure  and  neurobiotactic 
influence.  External  pressure  is  exerted  on  three  sides :  from  above  by 
the  downward  pressure  of  the  neopallium  (as  pointed  out  by  Elliot 
Smith,  Levi  and  many  other  neurologists),  medially  by  the  opposite 
hemisphere,  and  ventrally  by  the  rigid  barrier  of  the  dorsal  commis- 
sure and  subpallial  structures,  beneath  the  choroid  fissure  (the  atro- 
phic  choroid  barrier  so  strongly  emphasized  by  Levi,  1904).  The  in- 
ternal pressure  arises  from  the  intrinsic  growth  of  the  hippocampus, 
chiefly  the  ammon's  horn,  which  must  crumple  or  roll  to  adjust  itself 
to  the  space  allotted.  The  neurobiotactic  influence  is  in  the  first  place 
due  to  the  "perforating  fibers"  of  the  great  temporo-ammonic  tract 
and  fibers  associated  with  it  in  the  cingulum  limitans  and  cingulum 
ammonis  (see  pages  201  and  205).  These  fibers  distribute  equally 
to  the  gyrus  dentatus  and  the  ammon's  horn.  Those  fibers  which  dis- 
charge into  the  granule  cells  of  the  gyrus  dentatus  break  across  the 
hippocampal  fissure  as  it  deepens  and  cause  a  more  or  less  partial 
obliteration  of  the  fissure  by  a  secondary  fusion  of  its  two  lips.  They 
pull  the  sheet  of  granules  upward  through  its  entire  length  along  a  longi- 
tudinal axis  (nearly  median  in  Caenolestes},  so  that  in  cross  section 
the  gyrus  dentatus  describes  a  horseshoe  curve.  The  efferent  fibers 
of  the  gyrus  dentatus  (axons  of  the  granules,  "mossy  fibers"),  con- 
verging upon  the  terminal  lamina  of  the  ammon's  horn,  cause  a  break 
at  the  position  of  the  interpositio  medialis  (ip.tn.,  Fig.  31),  and  the  dis- 
placement of  the  lamina  terminalis  into  the  ventral  concavity  of  the 
gyrus  dentatus.  Figure  na,  b,  c  illustrates  diagrammatically  the  mode 
of  formation  of  the  definitive  mammalian  "hippocampal  figure"  in 
Caenolestes  under  the  three  influences  named.  The  arrows  indicate 
the  directions  in  which  the  force  is  applied  and  its  relative  strength. 

It  might  be  possible  to  invoke  also  Kappers'  ever  useful  concept 
of  neurobiotaxis  to  explain  the  obliteration  of  the  precommissural  rift 
or  fracture  in  the  hippocampal  formation  (*,  Figs.  12,  i?a)  noted  above 


2io  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

page  208)  as  the  possible  boundary  line  between  the  intra-  and  extra- 
ventricular  ammon's  horn;  the  extraventricular  ammon's  horn  alone 
receives  "mossy  fibers"  from  the  gyrus  dentatus,  a  circumstance  that 
might  be  held  to  account  for  the  rift  or  fracture  line  in  question;  but 
the  grand  pyramids  of  this  region,  according  to  Cajal,  send  recurrent 
collaterals  into  the  external  plexiform  layer  of  the  ammon's  horn 
(stratum  lacunosum)  which  rake  the  whole  extent  of  the  ammon's 
pyramids,  both  intra-  and  extraventricular,  and  so  tend  to  close  any 
gap  in  them.  Furthermore  the  afferent  commissural  fibers  in  the 
alveus  might  contribute  to  the  same  result,  as  well  as  to  the  forma- 
tion of  the  interpositio  lateralis,  by  a  general  neurobiotactic  compacting 
of  the  whole  line  of  pyramids.  More  than  this,  the  temporo-ammonic 
fibers  which  distribute  to  the  ammon's  pyramids  either  by  way  of  the 
superficial  plexiform  layer  external  to  the  stratum  lacunosum,  or  by 
way  of  the  alveus  (CajaPs  temporo-alvear  tract),  engage,  according 
to  Cajal,  not  only  the  intraventricular  but  also  some  of  the  nearer 
extraventricular  pyramids,  perhaps  most  of  them.  At  any  rate  it  would 
seem  as  though  the  initial  agent  in  the  production  of  such  dislocations 
as  those  considered  is  always  the  functional  activity  of  the  intrinsic 
structures  concerned,  and  not  the  mechanical  action  of  the  fibers  which 
may  later  not  only  occupy  the  break,  but  enlarge  it,  even  to  the  extent, 
conceivably,  of  interfering  with  the  function  which  originally  pro- 
duced it. 

The  dorsal  or  hippocampal  commissure  (c.d.,  Fig.  31)  exhibits  a 
narrower  and  denser  dorsal  portion,  the  psalterium  dorsale  (ps.d.), 
which  Cajal  (1911)  considers  the  commissural  path  of  the  crossed 
portion  of  the  temporo-ammonic  system,  and  the  wider  and  much  more 
diffuse  psalterium  ventrale  (ps.v.)  the  commissure  of  the  ammon's 
axons.  Many  cells  are  mingled  with  these  latter  fibers  (the  nucleus 
of  the  commissure),  so  that  on  the  whole  the  dorsal  commissure  in 
comparison  with  the  much  denser  anterior  commissure  seems  larger 
than  it  really  is.  The  precommissural  fornix  fibers  (f.prcom.,  Fig.  30) 
may  be  seen  passing  vertically  downward  in  front  of  the  commissures, 
and  above  the  psalterium  dorsale  the  dense  short  mass  of  the  fornix 
longus,  bending  downward  and  partly  interweaving  with  the  psalteri'um 
dorsale,  forms  the  "knieformiges  Biindel"  (Koelliker;  Livini,  1908); 
they  belong  to  the  median  striae  Lancisii  (Johnston,  1913).  The 
descending  columns  of  the  fornix  (c.for.,  Fig.  31)  collect  in  the  usual 
way  close  to  the  midline  as  deeply  stained  oval  bundles  above  the  an- 
terior commissure  and  pass  down  behind  it  into  the  hypothalamic  re- 
gion. Two  similar  oval  bundles  of  pale  fibers  located  between  the 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      211 

fornix  bundles  and  the  anterior  commissure  on  either  side  of  the 
inferior  recess  (V.».)  may  be  the  supracommissural  bundle  of  the 
stria  terminals  (Johnston's  bundle  4,  1923).  They  seem  to  be  receiv- 
ing more  lateral  fibers  disposed  in  the  same  diagonal  direction  as  some 
of  the  darker  fibers  entering  the  fornix  bundles.  I  cannot  tell  whether 
they  are  of  septal  or  hippocampal  origin,  or  both.  These  bundles  can- 
not in  my  sections  be  definitely  followed  laterally  into  the  stria  bed 
above  the  anterior  commissure.  There  is  also  the  possibility  that  they 
belong  to  the  stria  medullaris  system,  but  I  cannot  follow  them  to 
the  habenula. 

As  Johnston  (1913)  was  the  first  to  point  out,  marsupials  pos- 
sess well  developed  medial  striae  Lancisii  above  the  dorsal  commissure, 
a  level  at  which  full-bodied  hippocampus  also  exists,  proving  that  the 
lateral  and  not  the  medial  stria  of  the  indusium  are  the  vestigial  rem- 
nants of  degenerate  hippocampus  in  "callosal"  brains.  These  median 
striae  are  perfectly  clear  in  Caenolestes,  but  they  are  omitted  from 
the  reduced  sections  figured  here.  They  are  seen  as  cross  cut  fibers 
upon  the  dorsal  surface  of  the  hippocampal  commissure  and  as  vertical 
fibers  cutting  across  the  anterior  dorsal  commissure  fibers  and  partly 
interwoven  with  them  (see  knieformiges  Biindel,  page  210). 

In  connection  with  the  structures  under  discussion  it  may  be  well 
to  describe  the  form  taken  by  an  interesting  ventral  diverticulum  of 
the  superior  recess  (of  Elliot  Smith),  which  has  been  described  and 
figured  in  sagittal  section  of  Johnston  (1913)  for  the  Virginia  opos- 
sum (Fig.  i6b).  In  Caenolestes  this  subcommissural  pouch  (Fig. 
i6a)  runs  its  course  between  section  510  (Fig.  30  shows  the  beginning 
of  the  glial  mass  in  which  it  is  embedded)  and  section  528.  The  dor- 
sal recess  or  sac  bends  down  around  the  rostral  surface  of  the  dorsal 
commissure,  carrying  with  it  membranous  roof  tissue  which  hypertro- 
phies in  the  septum  between  the  commissures  and  in  the  pial  side  of 
the  precommissural  area  to  form  a  rather  thick  mass  of  glial  tissue 
between  the  medial  fornix  fibers.  This  mass,  composed  of  densely 
crowded  fine  pale  granules,  is  sharply  separated  from  the  nervous 
tissue  adjacent  to  it.  Rostrally  it  is  bifurcated  and  caudally  it  comes 
to  a  median  point  in  the  septum  beneath  the  dorsal  commissure.  It 
contains  a  lumen  of  similar  shape,  lined  with  ependyma,  which  ends 
caudally  as  a  median  recess  in  front  of  and  above  the  inferior  recess 
and  bifurcates  rostrally  to  end  in  two  smaller  diverticula  in  the  pre- 
commissural septal  walls.  In  Caenolestes  the  communicating  canal 
to  the  dorsal  recess  or  pouch  is  collapsed  and  obscured  in  dark-stained 
membranes  and  blood  vessels.  Figure  i6a  shows  a  reconstruction  of 


212  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

the  ventral  diverticulum  of  the  dorsal  recess  in  Caenolestes  in  the  hori- 
zontal plane  below  the  entrance  of  the  communicating  canal ;  figure  l6b 
the  sagittal  section  of  this  diverticulum  in  the  Virginia  opossum  from 
Johnston  (1913,  part  of  figure  35,  redrawn). 

The  middle  portion  of  the  hippocampus  (Figs.  32-37)  is  naturally 
less  interesting  than  the  two  extremities,  which  offer  more  develop- 
mental and  adaptational  clues,  and  it  therefore  calls  for  but  few  re- 
marks here.  With  respect  to  the  amount  of  gyrus  dentatus  exposed 
upon  the  median  surface  of  the  hemisphere,  Caenolestes  is  intermediate 
between  Perameles  and  Notoryctes,  a  fact  which  indicates  that  the 
neopallial  pressure,  apparently  the  chief  varying  factor  here,  exceeds 
that  in  the  latter  and  falls  below  that  in  the  former.  According  to 
Elliot  Smith  ( i895b,  Fig.  6)  Notoryctes  has  the  least  extensive  neo- 
pallium  found  among  mammals,  and  "in  no  other  animal  does  one 
find  the  simplicity  of  arrangement  which  the  hippocampus  of  Notoryc- 
tes presents,  an  appearance  which  recalls  the  foetal  hippocampus  of 
Perameles  or  Macro  pus."  (This  statement  refers  only  to  what  I  have 
called  the  "hippocampal  figure"  as  seen  in  cross  section  and  not  to 
the  development  of  the  temporal  pole  of  the  hippocampus,  which  will 
be  discussed  below.)  The  intermediate  condition  of  the  exposed  gyrus 
dentatus  in  Caenolestes  is  especially  interesting  in  view  of  the,  simi- 
larly intermediate  condition  of  the  cerebellum  in  Caenolestes. 

It  should  be  held  in  mind  that  the  more  caudal  sections  of  the 
hippocampus  become  progressively  more  tangential  to  the  structure 
itself  and  present  an  increasing  distortion  of  the  "hippocampal  figure" 
of  double  interlocked  arcs,  as  well  as  an  exaggeration  of  its  size.  We 
should  have  a  true  picture  of  the  hippocampus  only  if  we  could  make 
all  sections  vertical  to  the  hippocampal  axis,  which,  since  that  is  curved, 
would  make  them  radial.  The  sections  show  that  the  hippocampus  as 
a  whole  has  in  Caenolestes  not  yet  "turned  the  corner";  that  is  to 
say,  the  typical  "hippocampal  figure"  does  not  appear  twice,  like  an 
object  and  its  inverted  image,  in  any  one  cross  section,  as  it  does,  for 
example,  in  the  rabbit's  brain  (Fig.  2oa,  b,  c,  Winkler-Potter,  re- 
drawn) ;  or  once,  the  "mirror  image"  only,  as,  for  example,  in  the 
lion  (Fig.  21,  Elliot  Smith,  redrawn)  and  in  man.  In  Caenolestes 
the  gyrus  dentatus  alone  has  recurved  and  therefore  it  occurs  in  two 
places,  one  dorsal  and  one  ventral,  in  some  sections  (Fig.  39).  Neither 
the  fimbria  nor  the  ammon's  horn  has,  however,  recurved,  and  hence 
the  ventral  hippocampal  figure  of  double  interlocked  arcs  is  incom- 
plete and  the  temporal  hippocampus  is  practically  unrecognizable  at 
first  glance.  The  Virginia  opossum  has  advanced  one  step  farther  in 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      213 

the  recurving  process  by  the  formation  of  a  small  forwardly  directed 
pouch  on  the  rostral  face  of  the  ammon's  horn  at  its  ventral  border 
below  the  caudal  end  of  the  fimbria  (Fig  i8a-g,  cross  sections,  from 
unpublished  drawings,  Streeter,  redrawn;  Fig.  i8h,  reconstruction 
macle  from  same).  This  pouch  is  evidently  formed  in  response  to  the 
neurobiotactic  attraction  of  the  gyrus  dentatus,  which  has  here  grown 
forward  beyond  the  field  of  the  unrecurved  ammon's  horn.  This  change 
still  does  not  result  in  a  complete  "hippocampal  figure".  But  in  higher 
marsupials,  like  Hypsiprymnus  (Fig.  193,  b,  c,  Livini,  1908,  re- 
drawn), the  temporal  development  has  advanced  to  a  point  which 
allows  the  typical  hippocampal  figure  to  appear  twice  in  the  same 
section. 

In  Notary ctes  (Fig.  22,  Dart,  1920,  Fig.  13,  redrawn),  to  my 
great  surprise,  the  temporal  pole  of  the  hippocampus  is  apparently 
quite  as  well  developed  as  in  Hypsiprymnus,  and  considerably  better 
developed  than  in  Caenolestes  and  the  Virginia  opossum.  Notoryctes 
is  a  sightless  form  without  external  ears,  and  the  elongation  of  the 
hippocampus  and  great  amount  of  recurving  of  its  temporal  pole  is 
perhaps  to  be  regarded  as  compensatory,  in  view  of  the  absence  or 
extreme  reduction  of  the  vis'ual  and  auditory  systems.  The  median 
surface  of  the  hemisphere  (Elliot  Smith,  i895b,  Fig.  i)  has  a  peculiar 
peaked  appearance,  which  may  perhaps  be  due  to  the  elongation  of  the 
hippocampus  and  its  consequent  caudal  bowing.  The  pressure  of  the 
slightly  developed  neopallium  is  not  sufficient  to  cause  a  great  degree 
of  inrolling,  not  so  much  as  in  Caenolestes.  Ornithorhynchus  also  has 
an  elongated  slender  but  well  formed  temporal  pole  of  the  hippocampus 
(Elliot  Smith,  i896b,  Figs.  4,  5,  5",  pp.  472-3 — the  section  given 
through  the  "tail"  of  the  hippocampus  is  not  very  near  its  temporal 
end,  however).  In  this  case  the  elongation  of  the  hippocampus  and 
its  temporal  recurving,  which  is  pronounced,  is  probably  to  be  corre- 
lated with  the  very  great  size  of  the  neopallium  and  consequent  com- 
pression of  the  hippocampus.  I  do  not  know  the  condition  in  Echidna, 
but  should  expect  a  similar  situation  from  similar  conditions.  The 
temporal  pole  of  Perameles  is  also  unknown  to  me. 

In  the  rabbit's  brain,  a  "callosal"  one,  in  which  the  hippocampal 
commissure  is  caudally  displaced  by  the  corpus  callosum,  there  are 
also  two  typical  hippocampal  figures,  posed  in  opposite  directions, 
joined  by  the  tangentially  cut  fimbria  (Fig.  2Oa,  b,  c,  Winkler-Pot- 
ter,  1911,  pis.  XII,  XIII,  XIV,  redrawn).  Here  we  have  two 
exactly  identical  and  completely  detached  images  in  front  of  the  mid- 
dle of  the  fimbria  (Fig.  2oa  and  b).  I  do  not  know  whether  this  is 


214  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

the  case  also  in  Hypsiprymnus  (Fig.  ipb).  Livini  does  not  give  a 
section  showing  the  fimbria  in  two  parts.  The  more  rostral  section 
(Fig.  193)  shows,  however,  a  little  ventral  "island"  which  may  be  the 
projecting  tip  of  the  uncus,  and  between  this  and  the  next  caudal  sec- 
tion given  the  fimbria  may  appear  in  two  parts;  but  even  so  it  would 
probably  be  less  recurved  than  in  the  case  of  the  rabbit. 

In  the  case  of  the  higher  and  more  microsmatic  mammals,  as  in 
the  lion  (Fig.  21),  and  in  man,  the  dorsal  "figure"  disappears  as  the 
corpus  callosum  elongates  and  the  supracallosal  hippocampus  (lateral 
striae  Lancisii  and  the  accompanying  gray)  stretches,  and  only  the 
ventral  figure  persists  completely.  We  can  thus  easily  assemble  a  short 
series  which  will  adequately  illustrate  the  remarkable  complete  reversal 
of  the  hippocampus  in  the  course  of  phylogeny  by  the  agency  of  the 
combined  action  of  callosal  growth,  tremendous  neopallial  hypertrophy, 
and  the  anchoring  of  the  hippocampus  in  a  rostral  position  by  the 
pyriform  cortex,  which  is  itself  strongly  anchored  to  the  bulbar  forma- 
tion. The  tuberculum  and  the  amygdala,  as  they  diminish,  retain  their 
old  places  one  behind  the  other  between  the  bulb  and  the  temporal  end 
of  the  hippocampus,  and  medial  to  the  lateral  peduncular  gray  (lateral 
olfactory  gyrus)  and  pyriform  cortex.  The  neopallium  expands  enor- 
mously in  the  caudal  direction,  further  accentuating  the  hippocampal 
reversion.  The  pyriform  cortex  loses  its  distinctive  histological  char- 
acter and  assumes  a  progressively  more  neopallial  appearance,  until  the 
primate  condition  is  attained,  where,  as  "gyrus  hippocampi"  it  becomes 
histologically  practically  identical  with  the  neopallium  (save  in  the  pres- 
ence of  the  external  fibrillar  layer  in  its  anterior  portion).  The  pos- 
terior pyriform  region,  that  devoid  of  secondary  olfactory  fibers  and 
richly  supplied  with  non-olfactory  fibers,  begins,  even  in  the  Virginia 
opossum,  (Gray,  1924)  to  resemble  neopallial  cortex  rather  closely 
along  the  border  for  the  caudal  prolongation  of  the  rhinal  fissure.  In 
the  mouse,  as  Cajal's  (1911)  intensive  studies  show,  this  area  has 
developed  an  exceedingly  characteristic  histological  structure  of  its  own, 
and  assimilation  to  the  neopallial  cortical  pattern  has  not  apparently 
made  so  much  progress.  The  same  is  true  in  Caenolestes,  in  which  the 
boundary  between  the  neopallium  and  the  posterior  pyriform  area  is 
pretty  definite  (Figs.  41-42),  so  far  as  can  be  construed  from  frontal 
sections.  The  conditions  existing  at  the  temporal  extremity  of  the 
hippocampus  in  Caenolestes,  the  Virginia  opossum,  Hypsiprymnus  and 
the  rabbit  certainly  stress  the  dynamic  character  of  the  temporal  dis- 
placement of  the  hippocampus — "the  brain  is  not  4a  rigid  mosaic  of 
morphological  units  which  were  laid  down  in  the  primordial  vertebrate 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.     215 

and  thereafter  preserved  inviolate"  (Herrick,  1922,  p.  199).  The 
little,  rostrally  directed  ammon's  horn  pouch  in  the  Virginia  opossum 
is  clear  evidence  of  the  regulative  nature  of  the  changes  going  on.  In 
Caenolestes  the  forward  advance  of  the  temporal  gyrus  dentatus  has  not 
removed  it  from  the  unrecurved  ammon's  field,  but  in  the  Virginia 
opossum  this  would  have  happened  if  the  ammon's  horn  had  not 
responded  by  the  formation  of  the  pouch. 

It  is  to  me  rather  surprising  to  find  the  gyrus  dentatus  leading  in 
the  reversal  of  the  hippocampus  when  rostrally  it  appears  in  definitive 
form  tardily.  But  this  tardiness  is  perhaps  more  apparent  than  real, 
since  the  lower  portion  of  the  precommissural  hippocampus  is  in  mam- 
mals, as  Elliot  Smith  has  stated  for  the  same  region  of  the  posterior 
part  of  the  reptilian  hippocampus,  clearly  "on  the  way"  towards  differ- 
entiation into  definitive  gyrus  dentatus.  In  the  mammalian  precom- 
missural hippocampus  it  is  the  ventral  portion,  that  directly  contin- 
uous with  the  definitive  gyrus  dentatus,  which  takes  the  lead  in  differ- 
entiation. Differentiation  apparently  travels  from  below  upwards,  and 
the  definitive  ammon's  horn  or  rather,  perhaps,  its  "intraventricular" 
portion,  is  perhaps  the  last  to  develop  its  distinctive  structure  and 
extent.  It  is  at  least  the  last  to  begin  differentiating.  Pedro 
Cajal's  illuminating  studies  on  the  reptilian  brain  (Varanus  and 
Lacerta,  1917,  1919)  by  the  silver  methods  show  that  in  the  ventral 
or  medial  small-celled  portion  of  the  hippocampal  formation  (quasi 
gyrus  dentatus)  there  is  a  clear  transition  in  cell  type  from  the  deeper 
to  the  outer  cell  ranks.  The  cells  of  the  outermost  cell  ranks  (nearer 
the  pia)  are  practically  true  granules,  the  innermost  true  pyramids  of 
the  ammon's  type.  This  is  very  significant,  and  it  would  not,  I  think, 
be  surprising  to  find  in  full-bodied  precommissural  hippocampus  of 
lower  mammals  an  analogous  state  of  affairs.  The  situation  in  this 
Caenolestes  series  strongly  suggests,  far  in  advance  of  the  interpositio 
medialis  and  the  inrolling  of  the  gyrus  dentatus,  that  the  ventral  cells 
of  the  more  ventral  region  of  the  hippocampus  are  rapidly  verging 
towards  the  true  granule  type,  so  that  the  transition  to  definitive  gyrus 
dentatus  involves  apparently  no  sudden  cytological  changes.  The  hip- 
pocampus is  apparently  affected  by  two  distinct  waves  of  differentiation, 
both  starting  at  the  ventral  border  and  traveling  upward.  The  first  de- 
velops cells  of  pyramidal  type :  the  second,  following  in  its  wake,  trans- 
forms pyramids  into  granules  (phylogenetically  speaking).  The  first 
wave  affects  the  entire  width  of  the  hippocampal  formation,  reaching  its 
dorsal  border  (locus  of  the  interpositio  lateralis),  the  second  stops  at 
the  level  of  the  interpositio  medials,  affecting  the  "dentate"  region  but 


216  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

never  the  ammon's  region  above  it.  The  two  main  types  of  cells  are 
perhaps  to  be  interpreted  as  an  expression  of  the  enhancement  of 
function  in  the  hippocampus  by  its  organization  into  a  highly  inte- 
grated duplex  structure,  the  ammon's  horn  from  which  spring  the  long 
projection  axons  (which  alone  form  the  efferent  path  of  the  hippocam- 
pus), and  the  gyrus  dentatus,  whose  shorter  axons  of  a  specialized 
type  ("mossy  fibers")  deliver  stimuli  to  part  of  the  ammon's  pyramids 
directly  and  to  many  others  indirectly  (recurrent  collaterals  of  grand 
pyramids).  We  can  never  perhaps  get  the  complete  phylogenetic 
story  in  any  one  form,  certainly  not  in  the  lowest  and  highest,  where 
either  later  or  earlier  steps  fail  to  appear. 

Although  the  general  form  of  the  ammon's  horn  and  the  gyrus 
dentatus  cell  sheets  in  Caenolestes  is  really  simple,  it  is  difficult  to 
show  them  both  in  the  same  diagram.  Therefore  dissected  recon- 
structions (Figs.  i?a  and  i?b)  attempt  to  show  them  separately,  one 
or  the  other  being  cut  away  at  the  commissural  level  of  the  hippocam- 
pus. The  two  parts  are  shown  in  place  together  in  figure  12.  It  is 
seen  that  the  gyrus  dentatus  takes  the  form  of  a  sort  of  helmet  or 
hood  with  the  top  of  the  crown  or  apex  pointing  caudally;  the  front 
or  visor  of  the  helmet  corresponds  to  the  elongated  dorsal  portion  of 
the  gyrus  dentatus  extending  above  the  commissure  where  it  ends  in 
a  little  pouched  thickening  (Fig.  30)  ;  the  back  or  neck  portion  cor- 
responds to  the  ventral  or  temporal  recurved  part  of  the  gyrus  denta- 
tus. The  ammon's  horn  sheet  takes  the  form  of  a  loose  scroll,  greatly 
widened  posteriorly  in  the  sections  (Figs.  38-40),  so  that  it  is  really 
more  like  a  cornucopia,  with  the  wide  end  caudo-ventral.  The  up- 
turned median  flap  of  this  scroll,  covered  with  a  thin  coating  of  alveus 
fibers,  is  exposed  upon  the  median  surface  of  the  hemisphere,  save 
where  its  caudally  directed  corner  is  inserted  into  the  hooded  portion 
of  the  gyrus  dentatus.  This  upturned  ammon's  horn  forms  Elliot 
Smith's  extraventricular  or  inverted  hippocampus  ("dorsales  Blatt" 
of  Koelliker).  It  is  seen  in  the  sections  to  diminish  in  length  as  its 
lower  edge  recedes  upward  between  the  approaching  dorsal  and  ventral 
portions  of  the  gyrus  dentatus  (Figs.  34-40),  which  still  more  caudally 
unite  to  form  an  oval  ring  (the  "crown"  of  the  hood)  containing  the 
caudal  tip  of  the  inverted  hippocampus  (a  few  scattered  cells,  Fig. 
41).  At  the  caudal  end  of  the  choroid  fissure  the  continuity  of  the 
two  parts  of  the  ammon's  horn  (in  sections)  is  dissolved  (Figs.  40-42), 
and  the  lower  end  of  the  extraventricular  ammon's  horn  begins  to 
recede  upward,  while  that  of  the  intraventricular  ammon's  horn  seems 
to  turn  laterally  under  the  median  angle  of  the  ventricle  (Volsch's 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      217 

subventricular  hippocampus,  1906),  where  it  comes  in  contact  with 
the  amygdaloid  complex  (stria  bed  and  cortical  amygdaloid  nucleus). 
This  contact  extends  backward  along  the  line  of  the  medial  continua- 
tion of  the  amygdaloid  fissure  (fs.amg.m.,  Figs.  40-43),  always 
very  strongly  marked  internally  and  sometimes  externally  apparent. 
As  the  sections  show,  the  more  temporal  extremity  of  the  gyrus  den- 
tatus  (Fig.  39)  is  contiguous  with  the  ammon's  horn  and  perhaps  with 
the  amygdala  where  the  two  structures  adjoin.  The  fiber  connection 
between  the  hippocampus  and  the  accessory  basal  nucleus  of  the  amyg- 
dala (Johnston,  1923,  Fig.  55)  is  not  apparent  in  this  series.  This 
by  no  means  proves  its  absence. 

The  emphasis  here  placed  upon  the  amount  of  recurving  of  the  hip- 
pocampus is  intended  to  apply  specially  to  mammals,  where  it  is  to  be 
considered  in  connection  with  the  inrolling  of  the  hippocampus  under 
the  double  necessity  of  enhancement  of  function  and  economy  of 
space.  Even  mammals,  as  we  have  seen  above  (Notoryctes),  do  not 
present  a  quite  orderly  series  in  this  respect.  The  degree  of  recurving 
alone  is  not  a  criterion  of  advancement  in  hippocampal  development. 
In  some  reptiles  (e.g.,  Cistudo  Carolina  (Johnston,  1915,  Figs. 
6,  12,  13,  45)  the  recurving  of  the  temporal  pole  is  more  pronounced 
than  in  Caenolestes — in  single  cross  sections  two  separate  "hippocampal 
figures",  complete  for  this  brain,  appear.  The  factors  provoking  the 
recurving  in  such  cases  are  easily  recognisable  and  need  not  be  detailed 
here.  The  factors  operating  in  mammals  to  bring  about  inrolling  have 
apparently  provoked  a  method  of  increase  for  the  hippocampus,  which, 
in  concert  with  other  existing  conditions,  may  result  in  less  recurving 
of  the  temporal  pole  than  in  lower  forms.  In  higher  forms  the  caudal 
displacement  of  the  hippocampus  due  to  callosal  elongation,  reverses 
or  recurves  the  inrolled  hippocampus  exactly  as  in  the  case  of  simpler 
brains  alluded  to.  What  we  seem  to  have  always  before  us  is  the 
structural  record  of  the  solution  of  various  problems  of  regulative  be- 
havior, from  which  we  may  attempt  to  reconstruct  a  phylogeny  of 
function.  We  should  remember  that  there  are  always  a  great  number 
of  factors  working  in  concert  or  as  more  or  less  independent  variables, 
and  that  the  outstanding  potency  of  any  particular  factor  does  not 
mean  its  exclusive  activity. 

Above  the  medial  amygdaloid  fissure  (fs.amg.m.,  Figs.  6,  12,  17, 
40-43)  the  dorsal  or  subicular  border  of  the  ammon's  horn  adjoins,  as 
it  curves  upward,  the  posterior  pyriform  cortex  along  a  line  reaching 
upward  to  the  medial  extension  of  the  rhinal  fissure  (fs.rh.m.}  thus 
completing  the  great  horizontal  rhinencaphalic  cortical  ring,  as  de- 


218  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

scribed  in  the  earlier  part  of  this  paper  dealing  with  the  external  form 
of  the  brain  (page  185).  In  Caenolestes  this  line  is  longer  than  in  the 
Virginia  opossum,  because  the  position  of  the  rhinal  fissure  is  consider- 
ably higher.  The  exact  point  at  which  the  rhinal  fissure  hits  the  subic- 
ular  edge  of  the  hippocampus  is  not  determinable,  owing  to  the  fading 
out  of  histological  differentiation,  and  therefore  the  extended  portion  of 
the  fissure  is  shown  as  a  broken  line  (fs.rh.m.,  Figs.  6,  12,  17,  42). 

Elliot  Smith  (i895b,  p.  183),  in  discussing  the  hippocampus  of 
Notoryctes,  makes  the  following  remarks :  "The  hippocampus  is  equally 
convoluted  in  all  mammals,  because  it  reaches  its  maximum  develop- 
ment quite  early  in  the  phylogenetic  history  of  the  individual.  Thus 
in  Platypus  it  possesses  a  histological  differentiation  quite  as  complex 
and  fine  as  is  found  in  the  highest  mammals.  Like  the  pyriform,  it 
is  developed  early  both  in  phylogeny  and  ontogeny  in  accordance  with 
the  development  of  the  olfactory  apparatus.  Because  part  of  the  smell 
center  should  reach  a  high  state  of  development,  when  the  pallium  is  not 
proportionately  intricate,  is  no  argument  that  the  cortex  of  the  smell 
center  does  not  behave  like  the  rest  of  the  cortex  in  similar  circum- 
stances. It  should  be  noted,  however,  that,  intimate  as  is  the  connec- 
tion between  the  hippocampus  and  the  olfactory  lobe,  the  relative  sizes 
of  the  two  parts  are  by  no  means  constant.  Thus,  in  spite  of  the 
marked  difference  in  the  sizes  of  the  olfactory  bulb  in  Ornithorhynchus 
and  Perameles,  there  is  no  appreciable  difference  in  the  sizes  of  their 
hippocampi.  In  Notoryctes  the  size  of  the  hippocampus  is  relatively 
small,  considering  its  huge  olfactory.  What  determines  the  size  of 
the  hippocampus  is  hard  to  say."  (Italics  mine.) 

These  statements  are  very  suggestive,  and  in  my  study  of  the  cere- 
bral hemisphere  of  Caenolestes  they  have  often  recurred  to  me.  Are 
we,  however,  warranted  in  saying  that  the  hippocamp'us  reaches  its 
"maximum  development"  in  the  lower  mammals,  and  that  in  Ornitho- 
rhynchus the  histological  differentiation  is  "quite  as  complex  and  fine 
as  is  found  in  the  highest  mammals"?  It  is  true,  as  explained  above, 
that  the  definitive  mammalian  "hippocampal  figure"  appears  suddenly 
and  at  a  constant  level  in  the  lowest  mammals.  But  the  amount  of 
inrolling  and  the  development  of  the  temporal  pole  of  the  hippocampus 
both  vary  considerably,  as  we  have  seen,  in  the  marsupials  alone.  Now 
in  the  marsupials  we  find  also  a  great  variation  in  the  amount  of  neo- 
pallium  present,  without,  however,  a  greatly  elongated  eutherian  type 
of  corp'us  callosum  to  account  for  the  temporal  pole  by  mere  displace- 
ment of  the  entire  supra-  and  postcommissural  hippocampus.  Fur- 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.      219 

thermore,  in  considering  the  histological  differentiation  of  the  hippo- 
campus, we  now  know  that  if  we  take  into  account  not  merely  the 
simple  ranks  of  ammon's  pyramids  and  the  granules  of  the  gyrus  den- 
tatus  as  they  are  stained  by  ordinary  cell  methods,  such  as  Nissl,  but 
also  the  many  types  of  cells  in  the  other  layers  of  the  hippocampal 
cortex,  as  they  were  demonstrated  by  Cajal  (1911)  in  the  mouse  by 
the  silver  methods,  we  get  a  tremendously  suggestive  picture  of  the 
possibilities  for  progressive  complication  within  the  hippocampus.  I 
attempted  to  count  the  different  types  of  intrahippocampal  reenforcing 
and  stepping-up  devices  as  described  and  figured  by  Cajal  in  the 
mouse,  and  found  some  twenty-five  or  thirty  of  them.  Only  a  com- 
parative study  by  the  silver  methods  (such  as  that  being  carried  on 
by  del  Rio-Hortega,  1919)  of  the  whole  range  of  mammalian  brains 
could  reveal  the  stage  at  which  the  maximum  development  (maxi- 
mum histological  differentiation)  of  the  hippocampus  was  attained.  I 
should  rather  expect  to  find  it  nearer  the  upper  than  the  lower  end 
of  the  mammalian  phylum,  among  those  brains  in  which  the  discrep- 
ancy between  the  olfactory  bulbs  and  the  neopallium  is  marked,  but 
in  which  smell  is  still  an  active  function.  As  Elliot  Smith  remarks, 
the  hippocampus  does  not,  like  the  pyriform  lobe  and  the  tuberculum 
olfactorium,  decrease  pari  passu  with  the  olfactory  bulbs.  On  the 
contrary,  it  holds  its  own  (disregarding  the  amosmatic  brains,  and 
even  they  manage  to  retain  some  recognizable  hippocampus),  perhaps 
because  it  can  so  efficiently  combine  enhancement  of  function  with 
economy  of  space  (see  page  217)  as  the  neopallium  increases,  and 
this,  apparently  because  of  the  increasing  activity  back  and  forth  be- 
tween the  two.  In  this  connection  it  is  to  be  noted  (see  Herrick, 
1922,  page  196)  that  the  primordium  hippocampi  has  as  early  as  the 
preganoidean  stage  no  direct  somatic  connections,  and  in  the  ganoidean 
stage  begins  to  lose  direct  olfactory  connections,  and  very  soon  begins 
to  develop  a  characteristic  structure  in  the  presence  of  indirect  cor- 
relation and  associational  connections  from  various  sources,  "result- 
ing in  a  topographical  rearrangement  which  prepared  the  way  for  the 
differentiation  within  this  area  of  true  hippocampal  cortex  in  higher 
forms"  (Herrick,  1922,  page  196).  The  ability  of  the  hippocampus 
to  preserve  a  sort  of  structural  constancy  or  individuality  with  almost 
total  loss  of  direct  olfactory  connections,  and  in  the  face  of  great 
reduction  of  primary  and  secondary  olfactory  centers,  has  apparently 
undergone  no  abatement  in  the  presence  of  the  developing  mammalian 
neopallium.  Its  peculiar  structural  pattern  has  merely  unfolded  in  its 
own  way;  it  has  not  approximated  the  neopallial  structural  pattern. 


220  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

Functionally  the  increasing  neopallial-hippocampal  bond  is  of  tre- 
mendous import.  On  reflex  and  lower  psychic  levels  ol faction  is  mainly 
if  not  exclusively  linked  with  food  and  sex;  on  higher  psychic  levels 
it  becomes  increasingly  significant  esthetically  and  intellectually,  in 
correspondence  with  the  increase  of  the  neural  connections  between  the 
hippocampus  and  the  somatic  pallium  which  apparently  accompanies 
the  decrease  of  neural  connections  between  the  hippocampus  and  the 
diminishing  olfactory  bulbs  and  their  immediately  dependent  secondary 
olfactory  centers.  This  does  not  mean  that  the  hippocampus  increases 
phylogenetically  in  the  same  ratio  with  the  somatic  pallium,  or  in  any 
constant  ratio  with  it.  Apparently  neither  is  true.  But  it  seems  hard 
to  believe  that  a  direct  relation  does  not  exist  between  the  neopallium 
and  the  hippocampus,  a  relation  which  is  responsible  not  only  for  the 
remarkable  preservation  of  pattern  (not  its  initiation)  and  maintenance 
of  size  of  the  latter  structure  in  higher  and  relatively  microsmatic 
brains,  but  finally  for  the  sublimation  of  smell  into  a  "nobler"  sense, 
practically  as  truly  subservient  to  higher  psychic  life  as  are  vision  and 
audition.  Language  is  a  form  of  behavior,  and  the  rich  poetic  and 
spiritual  imagery  clustering  around  such  words  as  "fragrance"  and 
"incense"  amply  testifies  at  once  to  the  importance  of  the  existing 
sense  of  smell  in  man  and  to  its  sublimation.  So  too  does  the  tre- 
mendous evocatory  power  of  smell,  when  it  recreates  vanished  ex- 
periences which  arouse  high  and  beautiful  thoughts  or  emotions. 

CORPUS  STRIATUM 

The  corpus  striatum  consists  of  a  caudate  nucleus  (nuc.caud.,  Figs. 
12,  15,  28-38)  projecting  into  the  ventricle  and  extending  farther  ros- 
trally  and  caudally  than  any  other  part  of  the  striatum,  and  a  lenti- 
form  nucleus  lateral  and  ventral  to  it,  in  two  parts,  the  putamen  (put., 
Figs.  29-35)  and  globus  pallidus  (gl.p.,  Figs.  12,  15;  glob.p.,  Fig.  34) 
The  medial  or  septal  part  of  the  head  of  the  caudate  nucleus,  the  nucleus 
accumbens  (nuc.ac.,  Figs.  12,  20-30;  the  lateral  parolfactory  nucleus 
of  Johnston,  1913 — see  page  199),  has  already  been  mentioned  in 
connection  with  the  fusion  of  the  head  of  the  caudate  with  the  tuber- 
culum  (page  196).  Rostral  to  this  fusion  the  large  head  of  the 
caudate  is  laced  with  small  diagonal  fiber  bundles,  like  darning  stitches, 
and  heavily  fringed  on  its  ventral  border  with  diagonal  fibers,  all  the 
way  from  the  septum  to  the  lateral  arm  of  the  anterior  commissure 
(Fig.  29).  It  is  split  dorsally  by  a  small  caudally  directed  ventral  di- 
verticulum  of  the  lateral  ventricle,  roofed  by  the  lower  surface  of  the 
anterior  commissure  which  here  breaks  across  the  ventricle  to  reach 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.     221 

the  median  wall  (Figs.  30-31).  The  large  and  compact  internal  cap- 
sule (cap. i.,,  Figs.  29-34)  separates  the  caudate  nucleus  from  the  large 
putamen,  whose  lateral  boundary  is  the  curve  of  the  external  capsule 
(cap.e.,  Figs.  28-35).  The  caudate  nucleus,  including  the  nucleus 
accumbens,  and  the  putamen  are  in  this  series  characterized  by 
a  peculiar  dark,  gun-metal  gray  background,  seen  also  in  the 
lateral  and  basal  amygdaloid  nuclei,  setting  all  these  structures 
sharply  off  from  neighboring  ones.  I  do  not  know  what  it  may  be 
worth  as  a  criterion,  but  it  is  interesting  to  note  that  the  nucleus  accum- 
bens, which  Johnston  (1913)  considers  to  be  the  lateral  parolfactory 
nucleus",  is  uniform  with  the  rest  of  the  caudate  in  this  respect,  being 
sharply  delimited  from  the  septum  by  this  ground  color.* 

The  globus  pallidus  (gl.p.,  Figs.  12,  15;  glob. p.,  Fig.  34)  is  a  prom- 
inent mass  of  pale  giant  cells  in  a  rich  tangle  of  fibers,  situated  near 
the  center  of  the  sections  in  which  it  appears,  just  below  the  internal 
capsule  as  it  passes  into  cerebral  peduncle,  and  not  very  far  behind 
the  anterior  commissure  level.  Small  giant  cells  trailing  irregularly 
towards  the  basal  forebrain  bundle  in  the  innominate  region  (prethala- 
mus,  border  nucleus  of  Volsch)  may  belong  to  the  basal  nucleus  of 
the  palacostriatum  (De  Vries,  1910;  Kappers,  i92ib). 

NEOPALLIUM 

No  attempt  has  been  made  to  analyze  the  neopallium  of  Caenoles- 
tes  into  anatomical  regions,  although  even  this  inadequate  series  gives 
some  evidence  of  its  possibility.  It  is  hoped  that  the  two  series  to  be 
made  from  the  brains  of  Orolestes  may,  with  the  aid  of  the  intensive 
cortical  analysis  of  the  Virginia  opossum  in  this  laboratory  (Gray, 
1924),  render  a  fairly  adequate  analysis  of  the  somatic  cortex  possible 
in  Caenolestes  and  Orolestes. 

The  claustrum,  which  Brodmann  and  others  consider  to  be  a  neo- 
pallial  derivative,  is  certainly  present  in  the  more  dorsal  part  of  the 

*I  was  much  interested  to  read  (Kappers,  1923,  page  365)  of  the  "Spatz 
reaction"  of  sulphur  ammonium,  based  on  the  presence  of  iron,  which  gives  a 
much  stronger  blue  color  in  the  palaeostriatum  than  in  the  neostriatum.  While 
not  inclined  to  attribute  very  great  test  force  to  his  reaction,  Kappers  thought 
it  interesting  that  results  on  the  chick  and  man  corresponded.  In  this  iron- 
haematoxylin  series  of  Caenolestes  the  dark  coloration  has  a  very  different  dis- 
tribution. The  globus  pallidus  is  conspicuously  without  it.  The  caudate  nucleus 
(nearly  all  but  the  tail)  and  parts  of  the  tuberculum  fusing  with  it,  the  putamen, 
and  the  two  newest  amygdaloid  nuclei  (according  to  Johnston,  1923),  the  lateral 
and  the  basal,  are  the  regions  strikingly  affected  by  the  dark  coloration.  Very 
pronounced  in  the  rostral  portion  of  all  the  structures  named,  it  tends  to  fade 
out  caudally  in  each  of  them.  I  do  not  know  enough  about  either  the  Spatz 
reaction  or  the  dark  coloration  here  to  form  an  opinion  as  to  any  relation 
between  them. 


222  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

anterior  pyriform  region,  but  is  not  clearly  delimited.  It  apparently 
forms  an  irregular  cell  plate  between  the  external  capsule  and  a  cell- 
poor  strip  in  which  some  fibers  appear  which  may  be  an  incipient  cap- 
sula  extrema  (clau.,  cap. ex.,  Figs.  27-34).  Elliot  Smith  (19193,  b) 
considers  the  claustrum  a  derivative  of  the  upturned  lower  edge  of  the 
pyriform  cortex.  This  material  affords  no  real  evidence  either  way, 
although  the  claustrum  here  seems  to  be  continuous  with  the  deeper 
layers  of  the  neopallial  cortex. 

The  cells  of  this  brain,  which  are  remarkably  well  stained,  con- 
sidering the  method  used,  may  be  divided  first  of  all  into  two  groups, 
which  Volsch  (1906)  has  called  round  and  pyramid  cells,  referring 
to  the  hemisphere  only.  The  pyramidal  cells  (Using  the  term  very 
loosely  for  any  angular  cell)  are  black  and  show  dendritic  stumps, 
sometimes  very  long.  The  round  cells  are  pale-stained  with  the  nucleus 
clearly  visible.  The  variation  in  size  is  very  much  greater  for  the 
round  cells  than  for  the  pyramidal  cells.  All  the  giant  cells  of  the 
hemisphere  belong  to  the  former  type,  those  of  the  basal  olfactory 
region,  in  the  anterior  portion  of  the  lateral  limb  of  the  median  fore- 
brain  bundle  (see  page  198),  those  of  the  regio  innominata,  prethala- 
etc.  The  smallest  cells  also  belong  to  this  type,  the  olfactory  granules, 
those  of  the  gyrus  dentatus,  the  exceedingly  fine  granules  of  some  of 
the  islands  of  Calleja  in  the  tuberculum,  and  the  scattered  masses  of 
very  tiny  granules  tucked  between  adjacent  structures  like  packing. 
Volsch  considers  these  last  to  be  non-nervous,  I  do  not  know  on 
what  evidence  other  than  their  size  and  resemblance  to  stained  glia 
nuclei.  The  largest  cells  of  the  pyramidal  or  dark  angular  type  are 
probably  those  found  in  the  basal  nucleus  of  the  amygdala,  for  the 
hemisphere  at  least. 

GENERAL  CONSIDERATIONS 

In  the  beginning  of  this  study  of  a  type  of  brain  totally  unknown 
to  me  I  saw  that  no  rational  progress  was  possible  save  upon  a  broad 
and  strictly  comparative  basis.  In  the  course  of  parallel  studies  of 
the  literature  and  of  the  sections  of  this  particular  brain,  some  more 
general  considerations  arose,  first  as  persistent  questions,  usually  en- 
tirely unanswerable  by  me  at  least,  but  sometimes  coming  halfway  to 
rest  in  my  mind.  Two  of  these  are  briefly  outlined  below. 

One  of  the  most  puzzling  aspects  of  such  neurological  studies  as 
this  comes  out  in  connection  with  the  very  great  diversity  of  anatomi- 
cal pattern  of  structures  whose  afferent  and  efferent  connections  seem 
almost  or  practically  identical.  How  shall  we  explain  the  enormously 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.     223 

discrepant  anatomical  structure?  The  real  functional  adequacy  of 
every  existing  structure  must  of  course  be  taken  for  granted  as  the 
basis  of  its  existence.  But  more  than  this  must  apparently  be  called 
in  to  explain  its  particular  anatomical  type  of  structure.  Phyletic  tra- 
dition clarifies  some  things — inherited  type  of  anatomical  pattern  (or 
the  potency  to  develop  it)  retained,  embellished,  even  accentuated,  so 
long  as  it  is  functionally  utilisable  or  perhaps  not  actually  disadvantag- 
eous. The  hippocampus  is  perhaps  the  classic  example  under  this 
head.  Even  when  no  longer  adequate,  as  evidenced  by  reduction  and 
degeneration,  it  seems  very  hard  to  be  got  rid  of  entirely.  Fortunately 
for  the  comparative  neurologist,  it  hangs  on,  sometimes  only  a  pale 
and  shrunken  relic,  long  after  it  has  been  more  or  less  supplanted  by 
other  structures,  with  similar  or  unlike  functions,  which  have  become 
more  important  in  the  action-system  of  the  animal  either  because  of 
fundamentally  more  serviceable  type  of  anatomical  structural  pattern, 
or  because  of  more  useful  afferent  or  efferent  connections,  or  for  both 
reasons.  Thus  a  sort  of  natural  selection  (Roux's  "struggle  of 
parts"?)  is  constantly  operating  among  structural  patterns.  A  struc- 
tural pattern  is  safe  so  long  as  the  demands  upon  it  are  not  too  heavy, 
or  so  long  as  a  competing  pattern  with  greater  possibilities  does  not 
outrun  it.  The  superiority  and  final  supremacy  of  the  competing  pat- 
tern may  depend  not  only  upon  its  intrinsic  possibilities,  but  also  upon 
its  topographical  position  and  the  character  and  activity  of  neighbor- 
ing structures.  Thus  the  early  "physiological  isolation"  (to  use  Child's 
now  familiar  term,  1915,  1921)  of  the  primordium  hippocampi  from 
direct  sensory  stimuli  (see  page  219)  led  to  "topographical  and  physio- 
logical relationships  [which  have]  prepared  the  way  for  the  differentia- 
tion within  this  area  of  true  hippocampal  cortex  in  higher  forms" 
(Herrick,  1922,  page  196).  The  trail  of  the  Law  of  Neurobiotaxis 
is  to  be  seen  everywhere,  and  this  is  apparently  the  most  potent  single 
factor  which  can  be  invoked  immediately  to  "explain"  structural  pat- 
tern (i.e.,  to  translate  it  into  functional  pattern)  within  the  central 
nervous  system.  Acting  in  concert  with  phyletic  tradition,  it  might 
well  result  in  exaggerations  of  structural  pattern  which  do  not  seem 
to  parallel  the  development  of  functional  pattern.  We  must,  of  course, 
in  comparing  similar  lists  of  afferent  and  efferent  connections  of  very 
dissimilar  structures,  take  into  account  the  quantitative  and  positional 
differences  of  all  the  factors,  as  well  as  the  selective  operation  of  synap- 
ses. On  the  surface,  perhaps  the  discrepancies  seem  much  more  strik- 
ing on  the 'structural  than  on  the  functional  side,  although  sometimes 
the  reverse  seems  to  be  true.  We  must  of  course  believe  that  no  two 


224  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

structures  that  are  visibly  different  do  have  absolutely  identical  func- 
tions, not  even  right  and  left  members  of  paired  structures  in  the  same 
brain.  On  the  whole,  however,  the  physiologist's  frequently  voiced 
objection  to  the  anatomist's  too  great  emphasis  upon  structural  pattern 
may  not  perhaps  be  an  unjustifiable  one.  The  modern  anatomist,  of 
course,  regards  structure  as  merely  a  useful  and  convenient  clue  to 
function. 

Another  question  which  naturally  arises  in  such  a  study  as  this 
concerns  what,  if  any,  light  is  thrown  upon  the  method  of  progressive 
evolution  in  the  central  nervous  system.  Davidson  Black  (1913,  page 
366)  quotes  Roux's  definition  of  the  two  phases  of  the  development 
of  an  active  tissue:  "self-differentiation",  which  goes  on  without  re- 
gard to  functional  differentiation,  and  "dependent  differentiation", 
which  cannot  proceed  normally  in  the  absence  of  functional  connec- 
tions— in  the  nervous  system,  functional  continuity  of  neuron  systems, 
as  Bechterew  pointed  out.  The  transition  point  between  these  two 
phases  of  growth  Black  calls  the  "critical  point".  Apparently  the 
essential  character  or  activity  of  progressive  evolution  in  the  central 
nervous  system  is  the  pushing  forward  of  this  "critical  point",  relegat- 
ing more  and  more  of  the  period  of  "dependent  differentiation"  back 
into  the  period  of  "self-differentiation",  where  it  undergoes  compres- 
sion, abridgement,  and,  so  to  speak,  distortion,  thus  building  the  founda- 
tion for  newer  and  higher  development  in  the  period  of  dependent 
differentiation — like  using  the  capstones  of  a  newer  structure  to 
strengthen  and  enlarge  the  old  foundation  and  so  to  fit  it  for  still 
newer  and  more  ambitious  structures.  What  this  "antedating"  of 
structure  implies  in  physiological  terms,  what  metabolic  changes  or 
reorganization  by  which,  or  accompanying  which,  dissociations  take 
place,  some  phases  dropping  out,  others  being  temporally  dislocated 
with  reference  to  the  stimuli  formerly  necessary  to  elicit  them,  or 
whether  the  dissociations  are  more  apparent  than  real,  I  am  not  com- 
petent to  discuss. 

In  attempting  to  reconstruct  the  phyletic  history  of  any  form,  it 
is  to  be  remembered  that  not  all  the  foundation  blocks,  original  or 
"second-hand",  have  been  retained  in  the  enlarged  foundation — some 
have  been  rejected  entirely,  all  have  been  recut,  and  many  have  altered 
their  relations  with  reference  to  others.  The  higher  the  form  the 
greater  the  compression,  abridgment  and  rearrangement,  and  there- 
fore the  greater  the  necessity  of  prudence  and  hesitation  in  the  recon- 
struction of  the  detail  of  phyletic  pattern,  no  matter  how  simple  and 
obvious,  perhaps  deceivingly  so,  the  ontogenetic  pattern  may  appear. 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.     225 

SUMMARY 

1.  The  brain  of  Caenolestes  (and  of  Orolestes)  is  of  the  extreme 
macrosmatic  type,   characterized   by  the  great   size   of   the  olfactory 
bulbs  and  the  great  development  of  the  higher  rhinencephalic  centers 
(Herrick,  1921). 

2.  The  neopallium,  in  contrast,  forms  only  the  shallow  cap  of  the 
cerebral  hemisphere  above  the  high  rhinal  and  hippocampal  fissures, 
and  may  perhaps  be  relatively  the  least  extensive  or  the  second  least 
extensive  among  mammals  (Herrick,  1921). 

3.  The  olfactory  cortices — pyriform  and  hippocampal — form,  with 
the  aid  of  the  anterior  olfactory  nucleus,  two  periventricular  rhinence- 
phalic rings  partially  united  anteriorly :  a  smaller  quasi- vertical  one  ros- 
trally,   in  which  the  pyriform  and  hippocampal   cortices   are  doubly 
(supra-  and  infra ventricularly)  but  indirectly  united  through  the  an- 
terior olfactory,  nucleus ;  and  a  much  larger  horizontal  ring  formed  by 
the  additional  direct  union  of  the  posterior  pyriform  cortex  and  the 
subicular  margin  of  the  ammon's  horn  on  the  median  surface  of  the 
hemisphere  between  the  medial  prolongations  of  the  rhinal  and  amygda- 
loid fissures.    The  two  olfactory  cortices  are  split  apart  dorsally  by,  the 
vvedgelike  neopallium  and  ventrally  by  a  similar  wedgelike  formation 
composed  of  the  tuberculum  olfactorium  rostrally  and  the  amygdaloid 
complex  caudally. 

4.  The  marked  antero-posterior  foreshortening  of  this  brain  is  one 
of  its  characteristic  features,  frequently  in  evidence.     It  is  probably 
to  be  explained,  in  part  at  least,  by  the  exaggeration  of  the  subcortical 
centers. 

5.  The  hippocampus  begins  immediately  behind  the  olfactory  (bul- 
bar)  formation,  passes  through  practically  all  reptilian  stages  precom- 
missurally,  and  at  the  dorsal  commissure  rather  suddenly  assumes  defin- 
itive mammalian   form,   under   the   three-fold    influence   of    external 
pressure,  internal  pressure  and  neurobiotactic  attraction.    Its  temporal 
end  has  just  begun  to  recurve,  the  gyrus  dentatus  alone  being  involved. 
In  the  Virginia  opossum  the  ammon's  horn  has  begun  to  follow  suit. 
More  advanced  critical  stages  can  be  added  to  these  to  form  a  complete 
and  illuminating  series  of  the  phyletic  development  of  the  mammalian 
hippocampus,  omitting  the  monotremes  and  the  lowly  marsupial  Notor- 
yctes,  aberrantly  advanced  in  this  respect. 

6.  In  the  amount  of  gyrus  dentatus  exposed  upon  the  median  sur- 
face of  the  hemisphere  (an  index  of  the  degree  of  inrolling  of  the 
hippocampus)    Caenolestes  is   intermediate  between   Notorcyctes  and 
Perameles. 


226  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

7.  The  enormous  size  of  the  amygdaloid  complex  is  a  striking  char- 
acteristic of  this  brain,  as  is  also  its  prolonged  contact  with  the  temporal 
hippocampus,  both  ammon's  horn  and  gyrus  dentatus. 

8.  The  absence  of  the  aberrant  bundle  of  the  anterior  commissure 
would  on  the  basis  of  Elliot  Smith's  definition  of  it  as  a  diagnostic 
character  of  diprotodont  brains,  seem  to  put  Caenolestes  among  the 
polyprotodonts,  and  even  to  raise  the  question  of  the  exclusion  of  the 
diprotodonts  from  America. 

9.  The  cerebellum  of  Caenolestes  and  Orolestes  is  almost  exactly 
intermediate   between  those   of   Notoryctes   and   Perameles,   the   two 
simplest  mammalian  cerebella  hitherto  described. 

10.  In  consideration  of  the  marked  specialization  and  aberrancy  of 
the  two  monotreme  brains,  the  brain  of  Caenolestes  (and  of  Orolestes 
so  far  as  can  be  judged  from  the  external  anatomy)  ranks  as  one  of 
the  three  simplest  and  most  generalised  mammalian  brains  known  at 
the  present  time.    In  view  of  the  exaggerated  development  of  the  temp- 
oral pole  of  the  hippocampus  in  Notoryctes,  it  really  takes  first  rank. 
But  on  the  whole,  it  is  fairly  intermediate  between  Notoryctes  and 
Perameles,  the  two  simplest  and  most  generalized  mammalian  brains 
hitherto   described    (Elliot   Smith),   which   it  most   closely  resembles. 
It  should  offer — and  actually  does  offer — especially  promising  clues 
for  the  reconstruction  of  the  presumptive  phyletic  stages  involved  in 
the  transition  from  the  reptilian  to  the  mammalian  type.     And  since 
it  is  at  once  a  mammalian  brain  and  so  simple  and  generalized — almost, 
indeed,  a  mammalian  brain  reduced  to  lowest  terms — it  also  offers  hints 
of  Unusual  legibility  for  the  verification  of  the  structural  activities  of 
the  nervous  system,  viewed  as  regulatory  behavior  of  a  more  or  less 
plastic  material.     The  cerebral  hemisphere  of   Caenolestes,  the  only 
part  of  the  brain  yet  studied  in  any  detail,  fairly  swarms  with  exquis- 
itely clear  examples  of  structural  evidences  of  the  operation  of  neuro- 
biotaxis,  the  principle  whose  conception  and  definition  by  Dr.  Kappers 
of  Amsterdam  has  transferred  the  study  of  brain  morphology  from 
a  static  to  a  dynamic  basis. 

ACKNOWLEDGMENTS 

I  am  glad  to  acknowledge  here  my  indebtedness  to  Dr.  C.  Judson 
Herrick,  who  set  me  upon  this  research  and  who  has  aided  me  with 
practical  advice ;  to  Dr.  Wilfred  H.  Osgood,  who  procured  for  me  the 
use  of  further  material  and  in  other  ways  furthered  this  work;  to  Dr. 
Percival  Allen  Gray,  Jr.,  who  did  me  the  very  great  service  of  reading 
the  first  draft  of  this  paper  without  reference  to  the  figures,  and  offered 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.     227 

many  suggestions  which  have  greatly  contributed  to  the  lucidity  of 
the  final  draft;  to  Miss  Helen  Kates  for  welcome  assistance  in  the 
tedious  work  of  labelling  the  figures.  I  wish  it  were  possible  also  to 
acknowledge  fully  the  pleasure  and  benefit  I  have  received  from  all 
those  who  were  interested  enough  in  this  material  to  examine  or  dis- 
cuss it  with  me. 

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207,  Zool.  Ser.,  14,  pp.  157-162.    3  figures. 

I92ib.  The  connections  of  the  vomeronasal  nerve,  accessory  olfactory  bulb  and 
amygdala  in  amphibia.    Jour.  Comp.  Neurol.,  33,  pp.  213-280. 

1922.  Functional  factors  in  the  morphology  of  the  forebrain  of  fishes.    Libro 
en  honor  de  S.  Ramon  y  Cajal,  Madrid,  pp.  143-244. 

19243.    The  nucleus  olfactorius  anterior  of  the  oppossum.    Jour.  Comp.  Neurol., 

37,  PP-  3I7-359- 
I924b.     Neurological   foundations  of   animal  behavior.     New  York. 

VAN    H6EVELL,  J.   J.   L.   D. 

1916.  The  phylogenetic  development  of  the  cerebellar  nuclei.    K.  Akad.  van 
Wetenschappen,  Amsterdam,  18,  pp.  1421-1434. 

JOHNSTON,  J.  B. 

1913.    The  morphology  of  the  septum,  hippocampus,  and  pallial  commissures 

in  reptiles  and  mammals.     Jour.  Comp.  Neurol.,  23,  pp.  371-478. 
1915.     Cell    masses    in    the    forebrain    of    the    turtle,    Cistudo    Carolina.    Jour. 

Comp.  Neurol.,  25,  pp.  393-468. 

1923.  Further  contributions  to  the  evolution  of   the   forebrain.     Jour.   Comp. 
Neurol.,  35,  pp.  337-48i. 

KAPPERS,  C.  U.  ARIENS 

1917.  Further  contributions  to  neurobiotaxis.     IX.     An  attempt  to  compare 
the  phenomena  of  neurobiotaxis  with  the  phenomena  of  taxis  and  tropism. 
The   dynamic   polarization   of   the   neurone.     Jour.   Comp.   Neurol.,   27,   pp. 
261-298. 


228  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 

19213.  On  structural  laws  in  the  nervous  system.     The  principles  of  neurobio- 

taxis.     Brain,  44,  pp.  125-129. 
I92ib.  Die  vergleichende  Anatomic  des  Nervensystems  der  Wirbeltiere  und  des 

Menschen.      II.    Abschnitt:     Vergleichende    Anatomic    des    Kleinhirns,    des 

Mittel — und  des  Zwischenhirns  und  des  Vorderhirns.     Haarlem. 
1923.    Le   developpement  ontogenefigne   du  corps   strie   des   oiseaux  en   com- 

paraison   avec  celui   des   mammiferes   et   de   l'homme.     Schweizer   Arch.   F. 

Neurol.  u.  Psychiat.,  13,  pp.  348 — 370. 

LEVI,  G. 
19043.  Ueber    die    Entwicklung   und    Histogenese    der    Ammonshornformation. 

Schultze's  Arch.,  64,  pp.  389-404. 
i9O4b.  Sull'origine  filogenetica  della  formazione  ammonica.    Arch.  ital.  di  anat. 

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LIVINI,  G. 

1908.    II  proencefalo  di  un  marsupiale.     Arch.  ital.  di  anat.  e  embriol.,  6,  pp. 
549-584. 

McCOTTER,   R.   E. 

1912.    The  connections  of  the  votneronasal  nerve  and  the  accessory  olfactory 

bulb  in  the  opossum  and  other  mammals.    Anat.  Rec.,  6,  pp.  299-318. 
OBENCHAIN,  JEANNETTE  B. 

19233.    The  brain  of  Caenolestes  obscurus.    Anat.  Rec.,  25,  p.  145. 
192313.  The  brain  of  Caenolestes  obscurus.     Trans.  Illinois  State  Academy  of 

Science,  16,  pp.  100-106. 
OSGOOD,  W.  H. 

1921.    A  monographic  study  of   the  American  marsupial,   Caenolestes.     Field 
Mus.  of  Nat.  Hist.,  Pub.  207,  Zool.  Ser.,  14,  pp.  1-156. 

RAMON  y  CAJAL,  P. 

1917-19.    Nuevo  estudio  del  encefalo  de  los  reptiles.     Trabajos  del   Lab.   de 

Investigaciones  biol.  de  la  Univ.  de  Madrid,  15,  pp.  83-99;  J6,  pp.  309-333. 

1922    El  cerebro   de  los   batracios.     Libro  en   honor   de   S.   Ramon  y   Cajal, 

Madrid,  pp.  13-58. 
RAMON  Y  CAJAL,  S. 

1911.    Histologie  du  systeme  nerveux  de  l'homme  et  des  vertebres,  tome  II., 
Paris. 

RlO-HoRTEGA,   P.   DEL 

1919.     Particularides  histologicas  de  la   fascia  dentata  de  algunos  mamiferos. 
Trabajos  del  Lab.  de  Investigaciones  de  la  Univ.  de  Madrid,  16,  pp.  291-308. 

ROTHIG,  P. 

1910.     Riechbahnen,  Septum,  und  Thalamus  bei  Didelphys  marsupialis.     Abh. 
Senckenb.  Nat.  Ges.  Frankfurt  a.  M.,  31,  pp.  1-19. 

SMITH,  G.  ELLIOT 

1894.    A  preliminary  communication  upon  the  cerebral  commissures  of  mam- 
malia, with  especial  reference  to  the  Monotremata  and  Marsupialia.     Proc. 

Linn.  Soc.  New.  So.  Wales,  g,  pp.  635-656. 
18953.  The  cerebrum  of  the  marsupial  mole,  Notoryctes  typhlops.     Zool  Anz., 

1 8,  pp.  480-482. 
1895!).  The   comparative    anatomy    of    the    cerebrum   of    Notoryctes    typhlops. 

Trans.  Roy.  Soc.,  So.  Australia,  19,  pp.  167-193. 
18963.    The  brain  of   foetal   Ornithorhynchus.     Quart.  Jour.   Micr.   Soc.,   39, 

pp.  191-206. 
i896b.  Structure  of  the  cerebral  hemisphere  of  Ornithorhynchus.    Jour.  An3t. 

and  Physiol.,  30,  pp.  463-487. 

i896c.  The  fascia  dentata.    Anat.  Anz.,  12,  pp.  119-126. 
i896d.  The  morphology  of  the  limbic  lobe,  corpus  striatum,  the  reptilian  septum 

pellucidum,  and  the  fornix.     Jour.  Anat.  and  Physiol.,  30,  pp.  157-167,  and 

pp.  185-203. 
18973.  The  morphology  of  the  indusium  and  the  striae  Lancisii.     Anat.  Anz., 

13,  PP.  23-27. 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.     229 

18975.  The  origin  of  the  corpus  callosum;  a  comparative  study  of  the  hippo- 
campal    region    of    the   cerebrum    of    Marsupialia    and    certain    Cheiroptera. 
Trans.  Linn.  Soc.  Lond.,  7,  pp.  47-69. 

i897c.  Further  observations  upon  the  fornix,  with  especial  reference  to  the  brain 

of  Nyctophilus.     Jour.  Anat.  and  Physiol.,  32,  pp.  231-246. 
:897d.    The   fornix   superior.     Jour.   Anat.   and   Physiol.,   31,  pp.   80-92. 

18976.  The  relation  of  the  fornix  to  the  margin  of  the  cerebral  cortex.    Jour. 
Anat.  and  Physiol.,  32,  pp.  23-58. 

1899.  The  brain  in  the  Edentata.  Trans.  Linn.  Soc.  Lond.,  2nd  ser.,  Zool., 
7,  pp.  277-394. 

i9o2a.  Descriptive  and  illustrated  catalogue  of  the  physiological  series  of  com- 
parative anatomy  contained  in  the  Museum  of  the  Royal  College  of  Surgeons 
of  England.  Vol.  2.  London.  2nd  ed. 

i9O2b.  The  primary  subdivision  of  the  mammalian  cerebellum.  Jour.  Anat 
and  Physiol.,  36,  pp.  381-385. 

I9O2C.  Notes  on  the  brain  of  Macrosc elides  and  other  Insectivora.  Jour.  Linn. 
Soc.,  Zool.  Ser.,  28,  pp.  443-448. 

iox)2d.  On  a  peculiarity  of  the  cerebral  commissures  in  certain  Marsupialia 
not  hitherto  recognised  as  a  distinctive  feature  of  the  Diprotodontia.  Proc. 
Roy.  Soc.,  70,  pp.  226-231. 

19033.  On  the  so-called  "gyrus  hippocampi".  Jour.  Anat.  and  Physiol.,  37, 
pp.  324-328. 

i903b.  On  the  morphology  of  the  cerebral  hemisphere  in  the  vertebrata,  with 
especial  reference  to  an  aberrant  commissure  found  in  the  brains  of  certain 
reptiles.  Trans.  Linn.  Soc.  Lond.,  2nd  Zool.  Ser.,  8,  pp.  455-500. 

I9O3C.  On  the  morphology  of  the  brain  of  the  mammalia,  with  especial  refer- 
ence to  that  of  the  Lemurs,  recent  and  extinct.  Trans.  Linn.  Soc.  Lond.,  8, 
part  10,  pp.  319-432. 

i9X>3d.  Notes  on  the  morphology  of  the  cerebellum.  Jour.  Anat.  and  Physiol., 
37,  PP-  329-332. 

19036.  Further  observations  on  the  natural  mode  of  subdivision  of  the  mam- 
malian cerebellum.  Anat.  Anz.,  23,  pp.  368-384. 

19193.  A  preliminary  note  on  the  morphology  of  the  corpus  striatum  and  the 
origin  of  the  neopallium.  Jour.  Anat.,  53,  pp.  272-291. 

I9i9b.  A  note  on  Professor  Landau's  memoir  on  "The  comparative  anatomy 
of  the  nucleus  amygdalae,  the  claustrum  and  the  insular  cortex."    Jour.  Anat., 
53,  PP-  361-362. 
VOLSCH,  M. 

1906.    Zur    vergleichenden    Anatomic    des    Mandelkern    und    seiner    Nachbar- 

gebilde.    Anat.  Anz.,  68,  pp.  573-683;  ?6,  PP-  373-523- 
DE  VRIES,  E. 

1910.  Das  Corpus  Striatum  der  Saugetiere.    Anat.  Anz.,  37,  pp.  385-405. 
WINKLER,  C.  and  POTTER,  ADA 

1911.  An  anatomical  guide  to  experimental  researches  on  the  rabbit's  brain. 
Amsterdam. 

1914.  An  anatomical  guide  to  experimental  researches  on  the  cat's  brain, 
Amsterdam. 


230  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 


ABBREVIATIONS 


al. 

a.  In. 
amg. 

a.  prcom. 

a.  pt. 

aq. 
b. 

b.  ac. 

b.ol. 

b.  ol.  ac. 

c.  (Fig.  8) 
c. 

c.  a. 

c.  am. 
cap.  e. 
cap.  em. 
cap.  i. 

ch.,  ch.  op. 
ci.  am. 
ci.  lim. 
clau. 
c.  c. 
c.d. 

c.  g.  1. 

c.  g.  m. 

col.  a. 
col.  p. 
cort. 

cul. 

dec.  pyr. 
deep  nuc. 

em.  nat. 
ex.  am. 
ex.  am.  iven. 

ex.  am.  xven. 

ex.  dent, 
ex.  hip.  a. 


ex.  pir. 
ex.  pir.  a. 


alveus 

area   lunata  cerebelli 

nucleus  amygdalae ; 
amygdaloid  complex 

area  precommissuralis 

area  pteroidea  cere- 
belli 

aqueduct    of    Sylvius 

nucleus  basalis  amyg- 
dalae 

nucleus  basalis  acces- 
sorius  amygdalae 

bulbus    olfactorius 

bulbus  olfactorius  ac- 
cessorius 

brachium  conjucti- 
vum 

nucleus  centralis 
amygdadae 

commissura  anterior 
sive  ventralis ;  see  v. 

cornu  ammonis 

capsula   externa 

capsula  extrema 

capsula  interna 

chiasma  opticum 

cingulum   ammonis 

cingulum  limitans 

claustrum 

corpus  callosum 

commissura  dorsalis ; 
see  d.,  ps.  d.,  ps.v. 

corpus  geniculatum 
laterale 

corpus  geniculatum 
mediale 

colliculus    anterior 

colliculus  posterior 

nucleus  c  o  r  t  i  c  a  1  i  s 
amygdalae 

culmen,  pars  culmina- 
ta  cerebelli 

decussatio  pyramida- 
lis 

deep  nuclei  of  cere- 
bellum (undivided) 

eminentia    natiformis 

cortex  ammonis 

cortex  ammonis  intra- 
ventricularis 

cortex  ammonis  extra- 
ventricularis 

cortex  dentatus 

cortex  hippocampalis 
anterior  (precom- 
missuralis) 

cortex  piriformis 

cortex  piriformis  an- 
terior 


ex.  pir.  m. 

cortex  piriformis  me- 

dialis 

ex.  pir.  p. 

cortex  piriformis  pos- 

terior 

d.b. 

diagonal  band  of  Bro- 

ca 

d.v. 

diverticulum   ventralis 

(of      superior      re- 

cess) 

f  ila  ol. 

fila  olf  actor  ia 

fim. 

fimbria 

floe. 

flocculus 

f  .  med.  t. 

fasciculus  medialis  te- 

lencephali      (medial 

forebrain  bundle) 

f  .  med.  1.  1. 

fasciculus  medialis  te- 

lencephali      lateralis 

(lateral  limb  of  m. 

f.  b.) 

form.  bul. 

formatio  bulbaris   (ol- 

factorius) 

f.  prcom. 

fornix       praecommis- 

suralis 

f  s.  amg. 

fissura   amygdaloidea 

f  s.  amg.  m. 

fissura     amygdaloidea 

medialis 

fs.  circ. 

fissura  circularis 

fs.  ch. 

fissura  choroidea 

f  s.  di-tel. 

fissura     di-telencepha- 

lica 

f  s.  erh. 

fissura  endorhinalis 

fs.  f  .  d. 

fissura  fimbrio-dentata 

f  s.  fim.  al. 

fissura    fimbrio-alvea- 

ris 

f  s.  fim.  d. 

fissura  fimbrio-dentata 

f  s.  h.,  f  s.  hip. 

fissura   hippocampi 

fs.  pin. 

fissura  postlunata 

f  s.  pnod. 

fissura  postnodularis 

f  s.  orb. 

fissura  orbitalis 

f  s.  pr. 

fissura  prima  cerebelli 

f  s.  prcul. 

fissura  preculminata 

f  s.  rh. 

fissura  rhinalis 

f  s.  rh.  a. 

fissura    rhinalis    ante- 

f  s.  rh.  arc. 
f  s.  rh.  m. 
f  s.  s. 
f  s.  spyr. 
fs.  tr.  cb. 
g.  bas.  op. 

g-S 

gl.  p.,  glob.  p. 

gy.  dent. 


nor 

fissura  rhinalis  arcua- 
ta 

fissura  rhinalis  me- 
dialis 

fissura  secunda  cere- 
belli 

fissura  suprapyrami- 
dalis 

fissura  transversa  ce- 
rebelli 

ganglion  basale  op- 
ticum 

ganglion    semilunaris 

globus  pallidus 

gyrus  dentatus 


1925.       BRAINS  OF  CAENOLESTES  AND  OROLESTES — OBENCHAIN.     231 


hab. 
hip.  a. 

i.C. 
inf. 

interpos.  1. 
interpos.  m. 
int.  plate 

ip.  m. 
1. 


lob.  a.  cb. 
lob.  m.  cb. 
lob.  p.  cb. 
1.  perf  .  a. 
m. 

med. 
m.  i. 
neop. 
n.  2 

n.  5 
nod. 
n.  t.  o.  1. 

nuc.  ac. 
nuc.  amg.  b. 

nuc.  amg.  b.  ac. 
nuc.  amg.  c. 
nuc.  amg.  cort. 
nuc.  amg.  1. 
nuc.  amg.  m. 

nuc.  caud. 
nuc.  d.  b. 

nuc.  ip. 
nuc.  ol.  ant. 
nuc.  ol.  ant.  d. 

nuc.  ol.  ant.  ex. 
nuc.  ol  ant.  1. 


habenula 

hippocampus  anterior 
(  praecommissuralis  ) 

island  of   Calleja 

infundibulum 

interpositio  lateralis 

interpositio   medialis 

intercalary  plate  of 
Johnston  (1923) 

interpositio  medialis 

nucleus  lateralis 
amygdalae 

lingula  cerebelli 

lobus  anterior  cere- 
belli 

lobus  medialis  cere- 
belli 

lobus  posterior  cere- 
belli 

locus  perforatus  ante- 
rior 

nucleus  medialis 
amygdalae 

medulla   oblongata 

massa  intermedia 

neopallium 

nervus  opticus 

nervus  trigeminus 

nodulus  cerebelli 

nucleus  tracti  olfacto- 
rii  lateralis 

nucleus  accumbens 

nucleus  basalis  amyg- 
dalae 

nucleus  basalis  acces- 
sorius  amygdalae 

nucleus  centralis 
amygdalae 

nucleus  corticalis 
amygdalae 

nucleus  lateralis 
amygdalae 

nucleus  medialis 
amygdalae 

nucleus  caudatus 

nucleus  of  diagonal 
band  of  Broca 

nucleus  interpeduncu- 
laris 

nucleus  olfactorius 
anterior  (Her  rick) 

nucleus  olfactorius 
anterior,  pars  dorsa- 
lis 

nucleus  olfactorius 
anterior,  pars  exter- 
nus 

nucleus  olfactorius 
anterior,  pars  late- 
ralis 


nuc.  ol.  ant.  1.  v 
nuc.  ol.  ant.  m. 
nuc.  ol.  ant.  p. 

nuc.  pol.  1. 
nuc.  pol.  m. 
nuc.  tr.  ol.  1. 


ped.  floe. 

pfloc. 

pi.  ch.  3  (4) 

p.  prcul. 
p.  spyr. 

put. 
pyr. 
r. 

rec.  i. 
rec.  s 
rad.  thai. 
st.  glom. 
st.  gr. 
st.  m.  c. 

st.  med.. 

st.  t. 
st.  1. 1 


st.  t.  2 


st.  t.  3 


st.  t.  4 


st.  t.  5 


st.  t.  bed 
tect. 


nucleus  olfactorius 
anterior,  pars  late- 
ro-ventralis 

nucleus  olfactorius 
anterior,  pars  me- 
dialis 

nucleus  olfactorius 
anterior,  pars  pos- 
terior 

nucleus  parolfactorius 
lateralis 

nucleus  parolfactorius 
medialis 

nucleus  tracti  olfacto- 
rii  lateralis;  see  n. 
t.  o.  1. 

pedunculus   flocculi 

paraflocculus 

plexus  choroidea  ven- 
triculi  tertii  (quar- 
ti) 

pars  preculminata  ce- 
rebelli 

pars  suprapyramidalis 
cerebelli 

putamen 

pyramis  cerebelli 

corpus  restiforme 

recessus  inferior 

recessus   superior 

radiatio  thalamica 

stratum  glomeruli 

stratum  granulare 

stratum  cellularum  mi- 
tralium 

stria  medullaris,  see  st. 

*:  5 
stria  terminalis 

stria  terminalis, 
bundle  i  (Johnston, 
iQ23)=com.  bundle 

stria  terminalis, 
bundle  2  (Johnston, 
iQ23)=ol.  proj.  tr. 
(amg.) 

stria  terminalis, 
bundle  3  (Johnston, 
1923)  =subcom. 
bundle 

stria  terminalis, 
bundle  4  (Johnston, 
1923)  =  supracom. 
bundle 

stria  terminalis, 
bundle  5  (Johnston, 
1923)  =st.  med. 
bundle 

stria  terminalis  bed 

tectum   mesencephali 


232  FIELD  MUSEUM  OF  NATURAL  HISTORY — ZOOLOGY,  VOL.  XIV. 


t.ol. 

tr.  ol. 

tr.  ol.  d.  m. 

tr.  ol.  f  r. 
tr.  ol.  i. 

tr.  ol.  1. 
tr.  ol.  1.  d. 
tr.  ol.  1.  p.  d. 
tr.  ol.  1.  p.  v. 
tr.  ol.  1.  v. 

tr.  ol.  m. 
tr.  ol.  s.  +  c. 

tr.  op. 
tr.  st.  thai. 


tuberculum 
rium 


olfacto- 


tr.  t.  a. 


tractus   olfactorius 

tractus  olfactorius 
dorso-medialis 

tractus  olfacto-fronta- 
lis 

tractus  olfactorius  in- 
termedius  (commis- 
suralis) 

tractus  olfactorius  la- 
teralis (massive 
part) 

tractus  olfactorius  la- 
teralis, pars  dorsa- 
lis 

tractus  olfactorius  la- 
teralis, pedunculus 
dor  sal  is 

tractus  olfactorius  la- 
teralis, pedunculus 
ventralis 

tractus  olfactorius  la- 
teralis, pars  ventra- 
lis 

tractus  olfactorius  me- 
dialis 

tractus  olfacto-septa- 
lis  et  corticalis 

tractus  opticus 

tractus  strio-thalami- 
cus 


t.  v.  4 
uv. 
v.  cb. 
v.  1. 
v.  m.  a. 

v.  ol. 

v.  3  (4) 

xven.  alv. 
xven.  ex.  am. 


tractus  temporo-am- 
monis  of  C  a  j  a  1 
(1911;  sphenotemp., 
1906)  ;  see  ci.  lim. 
(cingulum  limitans) 
and  ci.  am.  (cingu- 
lum ammonis) 

taenia  ventriculi  quarti 

uvula  cerebelli 

ventriculus    cerebelli 

ventriculus   lateralis 

velum  medullare  ante- 
rius 

ventriculus  olfactorius 

ventriculus  t  e  r  t  i  u  s 
(quartus) 

extraventricular  alveus 

extraventricular  cor- 
tex ammonis 

line  of  fracture  sup- 
posed to  mark 
boundary  between 
extra-  and  intraven- 
tricular  precommis- 
sural  ammon's  cor- 
tex. 

arrows  to  show  direc- 
tion and  approxi- 
mate degree  of 
force  operating  in 
the  formation  of  the 
definitive  mamma- 
lian "hippocampal 
figure",  figure  u 

line  of  amputation  of 
cortex  dentatus,  fig- 
ure I7a 

line  of  amputation  of 
cortex  ammonis, 
figure  I7b 


FIELD  MUSEUM  OF  NATURAL  HISTORY. 


ZOOLOGY,   VOL.    XIV,   PL.  XXIV. 


fs'.rh. 


fs.  s.' 


OROLESTES  INCA. 
DORSAL  VIEW  OF  BRAIN. 

Five  times  natural  size. 


FIELD  MUSEUM  OF  NATURAL  HISTORY. 


ZOOLOGY,  VOL.  XIV,  PL.  XXV. 


•tr.  ol.lat. 
~.fs.rh.arc. 

erh. 


fs.  amg. 


OROLESTES  INCA. 
VENTRAL  VIEW  OF  BRAIN. 

Five  times  natural  size. 


FIELD  MUSEUM  OF  NATURAL  HISTORY. 


ZOOLOGY,   VOL.    XIV,    PL.   XXVI. 


f,5.  orb. 


-.  Vs.  erh. 
\\[''tr.  ol.lat. 

.  \  fs.  rfi.  arc. 
tub.  ol. 


fs  prcul. . 


fS.pnod 


v.  cb. 


OROLESTES  INCA. 

FIG.  3.    LATERAL  VIEW  OF  BRAIN. 

FIG.  4.    MEDIAN  SECTION  OF  BRAIN. 

Five  times  natural  size. 


FIELD  MUSEUM  OF  NATURAL  HISTORY. 


ZOOLOGY,   VOL.   XIV,    PL.    XXVII. 


hip.  a. 


MEDIAN  SURFACE  OF  HEMISPHERE. 

FIG.  5.    OROLESTES  INCA. 
FIG.  6.    CAENOLESTES  OBSCURUS.       RECONSTRUCTION. 

Five  times  natural  size. 


FIELD  MUSEUM  OF  NATURAL  HISTORY. 


ZOOLOGY,   VOL.  XIV,    PL.  XXVIII. 


prcul 


Tned. 


r        > 

V^_r/.v.c 

NOTORYCTES 

(Elliot  Smith) 

10  a 


10.  MEDIAL 


FIGS.    7  TO   10.     CAENOLESTES  OBSCURUS.         CEREBELLUM.          RECONSTRUCTIONS.         X  8. 

FIGS.   10a-10b.     CEREBELLA  OF  NOTORYCTES  AND  PERAMELES. 
FIG.    11.      DEVELOPMENT  OF  MAMMALIAN    "HIPPOCAMPAL  FIGURE. " 


o  o 
"  a 


FIELD  MUSEUM  OF  NATURAL  HISTORY. 


ZOOLOGY,   VOL.   XIV,    PL.   XXX. 


interpos.  lat.  (sxibic.) 


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FIG.    17.     CAENOLESTES  OBSCURUS.         DISSECTED  DIAGRAM  OF  HIPPOCAMPUS.         X   12. 
FIG.    -8.      DIDELPHIS  VIRGINIANA.         TEMPORAL  HIPPOCAMPUS.         X   5. 


FIELD   MUSEUM  OF  NATURAL  HISTORY. 


ZOOLOGY,   VOL.   XIV,    PL.   XXXI. 


cx  am 
ex.  dent. 


s.rk. 


M4  & 

CX.deni... 
fs.arng.  m.^T 


22 


TEMPORAL  RECURVING  OF  HIPPOCAMPUS. 

FIG.    19.     HYPSIPRYMNUS  (LiviNi)  X  3.  FIG.    20.     RABBIT  (WlNKLER-PoiTER). 

FIG.   21.     FELIS  LEO  (ELLIOT-SMITH)  X  Va.  FIG.   22.     NOTORYCTES  (DART). 


DIVERSITY  OF  Illinois  IIBBAH> 


FIELD  MUSEUM  OF  NATURAL  HISTORY. 


ZOOLOGY,    VOL.   XIV,   PL.   XXXII. 


b.  ol.  ac 
trollpd... 
nucolantl 

troll.. 
troll 

st.gr 

st  me., 
st.  pi  ex. 

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troll. 

nuc.olant.6K. 
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tr.ollpv.. 


neop. 


23  (SEC.  243) 


24  (SEC.  ZT1) 


t\uc  ol.ant.  d 
ex.  pit.  a 
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fs.  circ.. 
tr  ol.  I... 
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26(StC   445) 


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27  (5tC  3B2) 


a.  hi  p.  a. 
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fs.circ. 


.js.  circ. 
t.ol  U.C.) 


nucolontp. 
Vtrolm. 
/orm.  buL 


28   (SEC  4-10) 


CAENOLESTES  OBSCURUS. 
x  s. 


FIELD  MUSEUM  OF  NATURAL  HISTORY. 


ZOOLOGY,   VOL.   XIV,    PL.   XXXIII. 


,  cor  rod 

nuc.  caud. 


ex.  pir  a. 
trol.  Id. 


fs  erK 
troll..' 
ex.  pir  v. 
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fs.rkarc 


t.ol.  (i.C; 


(+  giant  cell  n 


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29  (SEC4J2) 


30  (Sec.  sio) 


ci.  lim. 


ex.  pir  a. 
trol.l.d. 
ex.  pir  v... 
fs.  erh 
trol.C. 
fs..Th.arc 
f.7ned.tl..- 
tn  ft  thai 


31  (SEC. 542) 


(Sec. 


CAENOLESTES  OBSCURUS. 
x  s. 


DIVERSITY  OF  RLWOISUBBW 


FIELD  MUSEUM  OF  NATURAL  HISTORY. 


ZOOLOGY,   VOL.   XIV,    PL.    XXXIV. 


C'L  llm. 


nuc.caadL 


cor.  rod 


cap. 
cap.i 
put. 
cap.e. 

cx.piTTn.lL..I  -    \ 
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. 

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33.  (Sec.  S9S) 


34:  (SEC.  642) 


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ci.am. 
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i 

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int  plate 
cap.  e. . 
nuc.ama.l 
nuc.amg.c. 
nucamg.m 


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is.  amg.- 
nuc.tr  ol.  I. 

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35.  CSEC.668) 


36  (5EC.ro4) 


CAENOLESTES  OBSCURUS. 

X  8. 


FIELD  MUSEUM  OF  NATURAL  HISTORY. 


ZOOLOGY,    VOL.    XIV,    PL.    XXXV. 


cor  rod 


frol.l.d.. 

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cx.plr  m 

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nuc.  amg.b. 

cap.  e 


. 

I.  perf.  a, 
nuc.tr.ol.  I 


37    (SEC.T30) 


38  (Sec. 760) 


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ci.am. 


cx.pirm 


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nuc.aTng  l./X-.'.': 
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true,  amg.b.  ac. 

(s.  aing 


4-0    (SEC.83Z.) 


CAENOLESTES  OBSCURUS. 
x  8. 


FIELD  MUSEUM   OF  NATURAL  HISTORY. 


ZOOLOGY,   VOL.   XIV,    PL.    XXXVI. 


Job.  m.cb.  (p.spyr) 
P-  Krh. 


nuc.o-mg.b.ai 

f  s.  amg 
nuc.amg.  cort. 


41  (SEC.  851) 


42  (SEC  879) 


fs.  pr 


fs.tr  cb. 
a.pt.cb. 


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{s.  prcul. 
fs.tr  cb... 


a.  PIT  p. 


43  (Stc.  900) 


44  (SEC.920) 


CAENOLESTES  OBSCURUS. 
x  s. 


UNIVERSITY  OF  ILLINOIS-URBANA 


